SR 666. Use of SUDS in High Density Developments Guidance Manual. RBB Kellagher CS Laughlan. Report SR 666 Release 4.

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1 SR 666 Use of SUDS in High Density Developments Report SR 666 Release 4.0 October 2005 RBB Kellagher CS Laughlan

2 Document Information Project Report title Client Client Representative Project No. Report No. Doc. ref. Project Manager Project Director Use of SUDS in High Density Developments DTI Russ Wolstenholme, WS Atkins MAS0437 SR 666 Environment Agency R&D Technical Report P2-261/18 SR666-SUDS rel 4-0 csl rk4.doc CS Lauchlan RBB Kellagher Document History Date Release Prepared Approved Authorised Notes 25/01/ rbbk Draft for consultation 14/03/ rbbk Final draft 02/06/ rbbk Final 02/06/ rbbk Revision of Appendix 3 Prepared Approved Authorised HR Wallingford Limited This report is a contribution to research generally and it would be imprudent for third parties to rely on it in specific applications without first checking its suitability. Various sections of this report rely on data supplied by or drawn from third party sources. HR Wallingford accepts no liability for loss or damage suffered by the client or third parties as a result of errors or inaccuracies in such third party data. HR Wallingford will only accept responsibility for the use of its material in specific projects where it has been engaged to advise upon a specific commission and given the opportunity to express a view on the reliability of the material for the particular applications. SR 666 ii R. 4.0

3 Summary Use of SUDS in High Density Developments Report SR 666 October 2005 It is accepted that conventional piped drainage systems exacerbate flood risk, detrimentally impact on water resources and associated ecology, and have contributed to the deterioration in the quality of receiving water bodies. The use of sustainable drainage systems (SUDS) to manage surface water runoff from new development is therefore now required in order to meet current planning and regulatory policy. This guide has been developed to assist developers, their professional advisors and local planning authorities with achieving drainage best practice on all new developments, with specific direction on achieving sustainable drainage solutions for high density sites. The document supports drainage infrastructure planning, conceptual design and construction, and provides guidance on: Stormwater management principles; Drainage design criteria; SUDS component characteristics; SUDS component applications; SUDS construction issues; Safety, amenity and environmental objectives; Existing drainage responsibilities and ownership. Barriers to the implementation of SUDS to date include a lack of clear design and construction guidance, a perceived disparity between planning policy on housing densities and the space required for surface drainage infrastructure on a development site, and the difficulties of securing adoption of SUDS systems to ensure long-term operation and maintenance. This document was conceived to provide specific assistance for developers on these issues with a focus on meeting the drainage needs for high density sites in particular. More detailed information on all aspects of SUDS will become available shortly with the publication of national technical guidance from CIRIA (due mid 2006). In Scotland, the Water Bill 2005 has introduced SUDS into legislation which allows Scottish Water to take over responsibility for them. In England and Wales, the options for the future management of SUDS are being reviewed by Defra; in the interim it is hoped that this document will provide clarification of a range of issues and thus facilitate discussions between stakeholders. SR 666 iii R. 4.0

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5 Acknowledgements This has been developed as part of DTI Partners in Innovation Contract CI 39/3/711 C2425. The objective of the project was to produce a guidance manual for use in evaluating and designing SUDS components, particularly for new developments where there are limitations on space availability for SUDS systems. The project work was undertaken by HR Wallingford under project number MAS0437. The Project Steering Group, which guided the work by HR Wallingford, represented a broad range of stakeholders. The names of the steering group members are listed below. Andrew Shuttleworth Barry Winter Brian D Arcy David Balmforth David Brook John Blanksby Jon Offer Katherine Robinson Kevin Ritter Neil McLean Nick Price Paul Shaffer Peter Martin Phil Chatfield Richard Ashley Richard Burrows Steve Evans Steve Wilson SEL Environmental Ltd Environment Agency SEPA MWH Ltd ODPM Pennine Water Group Westbury Homes Formpave Ltd Formpave Ltd SEPA Waterman Group Ltd CIRIA Black & Veatch Consulting Environment Agency University of Bradford University of Liverpool Thames Water Utilities (presenting UKWIR) Environmental Protection Group Ltd SR 666 v R. 4.0

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7 Glossary Attenuation Attenuation storage Brownfield Curtilage Design storm Detention basin Detention ponds District distributor roads Duration Extended detention basins Filter drains/trenches Filter strip Greenfield Greenfield runoff Green roof Infiltration basin Infiltration trench In/off line underground storage Slowing down the rate of flow, with a consequent increase in the duration of the flow. Temporary storage required to reduce the peak discharge of a flood wave Land which has already been built on in the past. Often associated with land that is contaminated. Land area within property boundaries A storm with a specified profile, intensity and duration. A basin, normally dry, constructed to store water temporarily to attenuate flows and provide some treatment. Ponds similar to Retention ponds, but with a permanent pool volume which is sized to provide a treatment volume of 1xV t. These roads distribute traffic between the residential, industrial and principal business districts of a town and form the link between the primary road network and the roads within residential areas. The time period over which an event occurs. A detention basin designed to retain runoff for an extended period to provide a greater degree of treatment. The hydraulic control requirements to achieve extended detention may be a limiting issue. A linear drain consisting of a trench filled with a permeable material, often with a perforated pipe in the base of the trench to assist drainage. Gently sloping vegetated areas designed to drain surface runoff as sheet flow from impermeable surfaces and remove sediment. Land that has never been developed, other than for agricultural or recreational use. The runoff rate and volume from a site prior to development. A vegetated roof surface that provides some retention of rainwater and promotes evapotranspiration. A basin hat is normally dry that is designed to store and infiltrate surface runoff into the ground. A trench, designed to infiltrate surface water into the ground. Underground voids, often constructed using concrete, grp, steel tanks or plastic void formers. They provide hydraulic attenuation, but do not treat the runoff. SR 666 vii R. 4.0

8 Glossary continued In-property storage Interception storage Linear ponds Local distributor roads Long term storage Mini Swale Pervious pavement Rainfall ratio r Residential access roads Retention ponds Return period Soakaway Standard Swale Temporary storage Treatment storage Storage of rainfall within the curtilage of the property, e.g. in the gutter, roof space or chamber. Provision of storage to prevent runoff from rainfall of up to 5mm, in order to replicate site responses for small rainfall events. An alternative to swales in serving runoff from roadways and hard standings. They are vegeatated open channels which provides storage, conveyance and some treatment. They are particularly useful in flat terrain. These roads distribute traffic within districts. In residential areas, they form the link between district distributors and residential roads. Storage of stormwater which is normally drained by infiltration that specifically addresses the additional volume of runoff caused by the development compared to greenfield runoff. The objective is to minimise any increase in flooding in the river downstream. Small, grass-lined, channels designed to store and treat runoff from small rainfall events and, when full, discharge excess water into the local drainage network. A permeable hardstanding designed to promote infiltration of surface runoff into a permeable sub-base. The ratio of depths for a 5 year return period event of 60 minute duration and a 5 year return period event of 2 days duration. These roads link dwellings and their associated parking areas and common open spaces to distributor roads. A pond where runoff is detained to allow settlement and biological treatment of some pollutants, as well as attenuate flows. Sized to provide a permanent pool volume of 4V t, where V t is the treatment volume. This indicates the average period, in years, between events of the same intensity or of a greater intensity than a particular event. A subsurface structure into which water is conveyed. They are designed to dispose of surface water by infiltration into the ground. A grass-lined channel designed to convey surface water, as well as controlling and treating the flow. Storage of the excess water for high intensity events, where the drainage system is temporarily overloaded. Storage provided to enable poor water quality to be improved by sedimentation and other treatment processes. SR 666 viii R. 4.0

9 Glossary continued Under-drained Swale Wetlands A grass-lined channel designed to store water for rainfall events. The stored water percolates down through the base of the swale into a perforated pipe that then discharges the flow into the local drainage network. A pond that has a high proportion of emergent vegetation in relation to open water and usually has a requirement for a continuous base flow. SR 666 ix R. 4.0

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11 Abbrieviations API 5 CEH CIRIA Defra DN Antecedent Precipitation index (over the previous 5 days) Centre for Ecology and Hydrology Construction Industry Research and Information Service Department for Environment, Food and Rural Affairs nominal size (DN) - a numerical designation of size which is a convenient round number equal to or approximately equal to the (pipe) bore in millimetres FEH Flood Estimation Handbook (Centre for Ecology and Hydrology (CEH), 1999) FSR Flood Studies Report (Institute of Hydrology, 1975) FSSR Flood Studies Supplementary Reports (Institute of Hydrology, ) IF M 5 60 NAPI PF PIMP PPG25 PPG3 PR Effective Impervious Area Factor The depth (in mm) for a 5-year return period event of 60 minute duration. New Antecedent Precipitation Index Porosity fraction (soil storage depth) Percentage Impermeable proportion of a catchment or subcatchment contributing to runoff Planning Policy Guidance 25 Development and Flood Risk, applicable to England Planning Policy Guidance 3 Housing, applicable to England Percentage Runoff SAAR Standard Average Annual Rainfall ( ) SOIL SPR SUDS V t WRAP Soil type classification used by Institute of Hydrology, FSR, 1975 and the HR Wallingford and Institute of Hydrology, Wallingford Procedure, 1981 Standard Percentage Runoff. Used in FSR and FEH equations as a soil runoff performance measure. Sustainable Drainage Systems The treatment volume calculated to retain the runoff from 90% of all events. Winter Rain Acceptance Potential, from the FSR. A classification used to indicate the runoff characteristics of the soil type. SR 666 xi R. 4.0

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13 Contents Title page Document Information Summary Acknowledgements Glossary Abbrieviations Contents i ii iii v vii xi xiii 1. Introduction and Scope Introduction Scope Stormwater Management Principles Drivers for sustainable drainage Sustainable drainage systems (SUDS) Design Criteria General Flood protection criteria Climate change criteria Water quality Operation and maintenance SUDS Descriptions and Characteristics Introduction General issues for all SUDS components using infiltration In-property SUDS (Water Butts, storage for domestic water use) Pervious pavements Filter trenches Filter strips Swales Ponds Linear ponds Wetlands Detention basins Soakaways and infiltration trenches Infiltration basins Green roofs Bio-retention Summary of SUDS performance characteristics Application of SUDS Development land use Integrated design In-curtilage areas Roadways Public open space Construction Issues Construction advice for pervious pavements...62 SR 666 xiii R. 4.0

14 Contents continued 7. Drainage design and performance assessment Introduction Design process Level of service flood protection System robustness Operation and maintenance Storage to meet discharge consents Refinement of storage volume and system performance assessment Future additional measures of sustainability of drainage systems Safety, Amenity and the Environment Introduction Amenity Environment Safety Liability Summary Stormwater Ownership and Management Drainage responsibilities and ownership Local authority drainage responsibilities Environmental regulators drainage responsibility Sewerage undertakers Highway authorities drainage responsibilities Home owner / private (In-curtilage) SUDS ownership Conservation organisations drainage responsibilities Summary of SUDS adoption by drainage organisations References Tables Table 2.1 Stormwater runoff comparison Development / Greenfield site...3 Table 2.2 Water quality impacts of development...4 Table 2.3 The benefits of SUDS...6 Table 2.4 Removal mechanisms for each pollutant category...8 Table 2.5 SUDS techniques - their hydraulic, water quality and amenity capabilities...9 Table 3.1 Drainage design criteria for flood protection...12 Table 3.2 Current best practice Climate Change and other safety factors...13 Table 3.3 Water quality treatment methods and suitable SUDS components...13 Table 3.4 River morphological criteria...14 Table 3.5 Self-cleansing velocity criteria for pipes serving a SUDS drainage system...15 Table 4.1 In-property SUDS characteristics for different categories...19 Table 4.2 Pervious pavement characteristics for different categories...21 Table 4.3 Filter trench characteristics for different categories...23 Table 4.4 Filter strip characteristics for different categories...25 Table 4.5 Swale characteristics for different categories...27 Table 4.6 Pond characteristics for different categories...29 Table 4.7 Linear pond characteristics for different categories...32 Table 4.8 Wetland characteristics for different categories...34 Table 4.9 Detention basin characteristics for different categories...36 SR 666 xiv R. 4.0

15 Contents continued Table 4.10 Soakaway and infiltration trench characteristics for different categories...38 Table 4.11 Infiltration basin characteristics for different categories...40 Table 4.12 Green roofs characteristics for different categories...42 Table 4.13 Bio-retention characteristics for different categories...44 Table 4.14 Summary table of performance characteristics for SUDS components...46 Table 5.1 Functions of public open space in PPG 17 (ODPM, 2002)...58 Table 7.1 Interception volume for a typical development...72 Table 7.2 Long term storage for a typical development...74 Table 7.3 SPR values for different SOIL values (from FSR)...74 Table 7.4 Approximate attenuation storage volumes required for a typical site for a 100 year event...78 Table 7.5 Recommended greenfield runoff rate estimation (SUDS Interim Code of Practice)...78 Table 7.6 SOIL Index descriptions (from the Flood Studies Report, Institute of Hydrology, 1975)...79 Table 7.7 Hydrological region factors for Q BAR...80 Table 7.8 Typical values of Q BAR for SAAR of 700mm and a site of 50ha...80 Table 8.1 Summary of safety, amenity and environmental factors and issues requiring compromise in design...91 Table 9.1 SUDS ownership/maintenance by drainage organisation status as of 2004/ Figures Figure 2.1 Pre and post development hydrological processes...4 Figure 2.2 The SUDS triangle reducing the impact of urban drainage on the environment...5 Figure 4.1 Water butt...19 Figure 4.2 Pervious pavement...21 Figure 4.3 Filter trench...23 Figure 4.4 Filter strip...25 Figure 4.5 Swales...27 Figure 4.6 Retention pond...29 Figure 4.7 Linear pond...32 Figure 4.8 Wetland...34 Figure 4.9 Detention basin...36 Figure 4.10 Soakaway...38 Figure 4.11 Infiltration basin...40 Figure 4.12 Green roof...42 Figure 4.13 Bio-retention area...44 Figure 5.1 Typical division of land use for new developments and opportunities for SUDS use...48 Figure 5.2 Decision guidance for use of appropriate SUDS components...50 Figure 5.3 The use of SUDS and infiltration...51 Figure 5.4 Possible location of SUDS components in a new development...52 Figure 5.5 Possible location of SUDS components in a new development...53 Figure 5.6 House frontage directly onto the road...54 Figure 5.7 A Home Zone road with pervious pavement...55 Figure 5.8 Communal parking areas...57 Figure 5.9 Linear pond...57 Figure 7.1 Schematic of the drainage design process...67 SR 666 xv R. 4.0

16 Contents continued Figure 7.3 Storage components for stormwater runoff control...73 Figure year 6 hour rainfall depths for hydrological FSR regions across the UK...76 Figure 7.5 Additional runoff volume caused by development where all pervious areas are assumed not to drain to the drainage network (Eqn. 7.2)...77 Figure 7.6 Additional runoff caused by developments where all pervious areas are assumed to drain to the drainage network (Eqn. 7.3)...77 Figure 9.1 UK sewerage undertakers...96 Boxes Box 6.1 Construction considerations for SUDS...61 Box 8.1 Invasive alien wetland plants which pose a high risk to the environment. These plants should be excluded from all SUDS planting schemes (HR Wallingford, 2003)...86 Box 8.2 Issues associated with planting SUDS ponds (HR Wallingford, 2003)...86 Appendices Appendix 1 Greenfield runoff estimation Appendix 2 Example of initial assessment of stormwater storage volumes Appendix 3 Attenuation storage volumes Appendix 4 Long term storage volumes Appendix 5 Flood Risk Assessments to accompany planning applications (England and Wales) Appendix 6 Review of relevant policy and guidance for SUDS Appendix 7 Research reports produced during this project SR 666 xvi R. 4.0

17 1. Introduction and Scope This chapter provides the context of this guide and who it is aimed at helping. 1.1 INTRODUCTION The Water Framework Directive (2000/60/EC) has been produced to address the environmental impact of man s water based activities, not least due to stormwater discharges. Over the last 2 decades, there has been a growing recognition of the negative impact of stormwater on receiving waters and the need to minimise its effect. The use of sustainable drainage systems (SUDS) is now recognised as essential in achieving this aim as they provide a range of methods/techniques for partially treating stormwater runoff. The Environment Agency (EA), Scottish Environmental Protection Agency (SEPA) (through NPPG 7 and PAN 61) and ODPM and Defra (through PPG25 in England) and Welsh Assembly Government (through TAN 15 in Wales) are making concerted efforts to get SUDS implemented in all new developments. In the UK there is growing government pressure to increase housing densities. Planning Policy Guidance note 3 was issued by DTLR (PPG 3, DTLR 2000) which emphasises the need for the footprint of developments to be minimised. The housing density suggested is up to 50 houses per hectare. This will create urban areas with a high proportion of impervious surface and therefore potentially restrict the opportunity to use certain SUDS components that often require more land than traditional drainage systems. It can be seen that there is a potential conflict between requirements of PPG3 and PPG25 and guidance is needed for Planners and Developers to maximise the effectiveness of SUDS systems within the constraints of meeting the target of providing high density developments. 1.2 SCOPE This guide provides a complete technical overview of all issues that should be addressed to achieve an effective drainage system. To ensure it is a useable document, this guide is concise and sources for more detailed information are referenced. This document addresses all issues from initial planning, through design, construction and adoption of SUDS. SR R. 4.0

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19 2. Stormwater Management Principles This chapter describes the SUDS philosophy and the necessity for using SUDS. A summary is provided of the principal hydrological and water quality impacts due to development. It details the advantages of using SUDS and provides an overview of the mitigation potential of each type of SUDS component. 2.1 DRIVERS FOR SUSTAINABLE DRAINAGE Traditional drainage The traditional method of draining stormwater from developments has been to use pipe systems. These are designed to prevent flooding locally by conveying the water away as quickly as possibly. This has the potential for exacerbating the flooding downstream, altering stream morphology and also polluting receiving water bodies with metals, hydro-carbons, BOD, nutrients and sediment. A more sustainable approach to drainage has been evolving to address these problems. This is referred to as SUDS (Sustainable Drainage Systems) in the UK and BMPs (Best Management Practices) in the USA and other countries The hydraulic impacts of development on stormwater The following section is a brief description of the hydraulic impact of development and a comparison is made with runoff from undeveloped areas. Development reduces the permeability of the land surface by replacing free draining ground with impermeable roofs, roads and other paved areas. Disposal of stormwater runoff from a development using a piped system results in a significantly different response from rain falling on a greenfield area. Table 2.1 summarises the differences in terms of hydraulic response. Table 2.1 Stormwater runoff comparison Development / Greenfield site Rainfall event Development response Greenfield response Small / frequent rainfall events Some runoff Rapid runoff River regime modified Highly polluted Little infiltration to support river base flows No measureable runoff All rainfall evapo-transpirated in summer No pollution in runoff Infiltration for river base flow support in winter Big / intense rainfall events High volume of runoff Rapid runoff Flooding exacerbated downstream Low to Medium volume of runoff Slow runoff Flooding not exacerbated Figure 2.1 illustrates these changes caused by development. SR R. 4.0

20 Courtesy of CIRIA Figure 2.1 Pre and post development hydrological processes The alteration of the flow regime in the river (both in terms of the total quantity of runoff and the peak runoff rates) leads to flooding and channel erosion downstream of the development. This also affects the bank-side habitats and the flora and fauna in the stream and results in a reduction of diversity and abundance. It is anticipated that climate change will exacerbate many of these issues, as summers are predicted to become drier, winters wetter and rainfall generally becoming more intense The water quality impacts of development on stormwater Man creates a large number of pollutants which become entrained in stormwater runoff. These include sediments, oils, grits, metals, fertilisers, pesticides, animal waste, salts, pathogens and litter. These cause environmental damage and can affect public health. In contrast, natural environments rarely cause any pollution from rainfall runoff (discounting some farming practices). The topsoil and vegetation provide a filtering mechanism for runoff, with most runoff occurring as inter-flow through the soil rather than as direct overland runoff. Table 2.2 summarises the impacts of development on runoff water quality. Table 2.2 Water quality impacts of development Water quality pollutants BOD (decaying vegetation and other organic material). Nutrient enrichment (e.g. raised nitrogen, phosphorous concentrations) Pathogen contamination (bacteria, viruses) Hydrocarbon contamination (e.g. Environmental Effects Oxygen depletion weakens stream life and affects the release of toxic gases from sediments deposited in the watercourse. Increased nutrient levels promotes weed, algal growth and excessive diurnal oxygen concentrations, eutrophication. Can result in fish kills. The concentration of bacteria and pathogens found in urban runoff can exceed public health standards for recreational water contact. Toxic materials (including hydrocarbons) kill aquatic organisms SR R. 4.0

21 Water quality pollutants TPH, PAH, VOCs, MTBE) Increased levels of toxic materials (e.g. metals, pesticides, cyanides) Sediment (particularly site construction) Litter Raised water temperatures Environmental Effects and accumulate in the food chain. Their presence increases drinking water treatment costs. As for Hydrocarbon contamination. Sedimentation and concretion affecting biota habitat, and stream erosion and deposition. Aesthetic impact, obstruction of flows and risks to wildlife. Oxygen depletion which weakens stream life and affects the release of toxic gases from sediments deposited in the watercourse 2.2 SUSTAINABLE DRAINAGE SYSTEMS (SUDS) The SUDS philosophy Drainage systems need to be developed in line with the ideals of sustainable development to minimise the anthropogenic impact by addressing the various factors that cause environmental stress. Surface water drainage methods that consider quantity, quality and amenity are collectively referred to as Sustainable Drainage Systems (SUDS). The philosophy of SUDS is to try and replicate the natural drainage of a site prior to development and to treat runoff as effectively as possible to remove pollutants, thus reducing the negative water quality impact on receiving water bodies. Quantity SUDS Quality Amenity and Bio-diversity Courtesy of CIRIA Figure 2.2 The SUDS triangle reducing the impact of urban drainage on the environment SR R. 4.0

22 Appropriately designed, constructed and maintained, SUDS are more sustainable than conventional drainage methods because they can mitigate many of the adverse effects of urban stormwater runoff on the environment. They achieve this through the processes listed in Table 2.3. Table 2.3 The benefits of SUDS Reducing runoff rates Reducing runoff volumes Reducing the frequency of runoff Increasing natural groundwater recharge Increasing dry weather flows in watercourses Reducing pollutant concentrations Contributing to the amenity value of the development Contributing to the aesthetic value the development Providing ecological habitats Reducing water supply demand when stormwater is reused To qualify as a SUDS component, drainage components should theoretically provide an element of each of the three benefits of hydraulic, water quality and amenity. However this would, by definition, preclude the use of pipes which is normally an integral part of any drainage solution. Rather than constrain the definition of what does or does not comply with being a SUDS solution, it is easier to consider the objective of any drainage scheme as being: To replicate, as closely as possible, the predevelopment runoff characteristics of a site for all rainfall events. This leads to the consideration of techniques which: 1. Maximise infiltration subject to economic, environmental and physical constraints. 2. Maximise the use of filtration and sedimentation and other treatment techniques to obtain water quality improvements. 3. Provide appropriate levels of attenuation to reduce the rate of runoff. 4. Consider issues of biodiversity, amenity and aesthetics where the opportunity occurs SUDS runoff management processes for stormwater There are several processes that should be used to manage and control the runoff from developed areas. Each SUDS component provides a mix of these features to manage the stormwater runoff. (a) Conveyance Conveyance is the transfer of surface runoff from one place to another. Issues to consider are: self-cleansing velocities SR R. 4.0

23 erosion hydraulic capacity (b) Attenuation Attenuation is the reduction in the rate of runoff, which is achieved by the provision of a hydraulic constraint, and the storage needed for temporary detention of runoff. Considerations include: head-discharge relationship of the control, including upstream and downstream water level constraints, and storage volume needed to meet the hydraulic criteria. (c) Infiltration This is the discharge of runoff into the ground. Apart from evaporation and depression storage (losses which are small), and rainwater harvesting use, this is the only means by which runoff volume can be reduced. Issues to consider are: capacity of the soil to receive infiltration, risk of soil saturation and the possible resulting instability on sloping ground, depth to groundwater, proximity of groundwater abstractions, creating or exacerbating outflows at springs risk of pollution mobilisation and transfer, risk of blinding due to sediment load, risk of settlement (in weakly cemented soils), (d) Rainwater harvesting This is the direct capture and use of runoff on site. Rainfall runoff can be stored for domestic use (e.g. flushing toilets), or irrigation of urban landscapes. Issues to consider are: size of storage (which is a function of the local hydrology and demand), guaranteed level of service to be provided, benefits provided in reducing stormwater runoff, benefits provided in reducing the demand for treated mains water, overflow arrangements (for large storm events), possible negative impacts on public health (e.g. mis-connections). (e) Filtration This is the reduction of sediment load by means of slow sheet flow through grass or other media. Issues to consider are: velocity and depth of sheet flow, headloss through filtration media, maintenance implications for long term performance of the system. (f) Sedimentation This is the settling of suspended solids by providing relatively quiescent conditions. Issues to consider are: retention time, flow velocity, SR R. 4.0

24 the risk of sediment re-mobilisation, maintenance and disposal requirements for long term performance of the system. (g) Other processes Other forms of water quality treatment (such as nutrient stripping, photolysis, volatilisation, microbial bio-degradation and adsorption) all take place to some degree depending on the SUDS component used. The removal mechanisms appropriate for each pollutant category are presented in Table 2.4. Table 2.4 Removal mechanisms for each pollutant category Pollutant Nutrients Phosphorous, Nitrogen Sediments Total suspended solids Hydrocarbons TPH, PAH, VOC, MTBE Metals Lead, Copper, Cadmium, Mercury, Zinc, Chromium, Aluminium Pesticides Chlorides Cyanides Litter Organic matter, BOD Removal Mechanisms in SUDS Sedimentation, biodegradation, precipitation, denitrification Sedimentation, filtration Attack by soil microflora, sunlight, filtration and adsorption Sedimentation, adsorption, filtration Biodegradation, adsorption, volatilisation None Evaporation, sunlight Trapping, removal during routine maintenance Filtration, sedimentation, biodegradation SUDS techniques There are a wide range of individual techniques that can be used to form part of a SUDS system. These can be broadly categorised on whether their primary use is considered to be pre-treatment, conveyance, source, site or regional controls, and can be ranked in a simple manner, based on their hydraulic and water quality performance potential. Table 2.5 categorises the main SUDS components in use in the UK. The list is not comprehensive there are other components (e.g. sand filters) and a range of proprietary drainage components, which can contribute to developing an effective drainage system. Details of each SUDS component are provided in Chapter 4. SR R. 4.0

25 Table 2.5 SUDS techniques - their hydraulic, water quality and amenity capabilities Amenity Water Quality Hydraulic Performance Key to symbols: Benefits Treatment Primary process Ecology Amenity Aesthetics Nitrification Uptake by plants Precipitation Volatilisation Biodegradation Adsorption Filtration Sedimentation Water Harvesting Infiltration Attenuation Conveyance Secondary process SUDS Technique SUDS Description Storage for irrigation or domestic use (toilets etc). Rainwater harvesting (Water butts etc) Pervious pavements Car parks (and potentially minor roads) and other paved surfaces which provide storage of rainwater in the underlying construction. Filter trenches Linear drains/trenches filled with a granular material, often with a perforated pipe in the base of the trench. Filter strips Vegetated strips of gently sloping ground designed to drain water using sheet flow from roads and parking areas. Swales Shallow vegetated channels that conduct and infiltrate runoff. The vegetation filters sediments. Ponds Areas of permanent water used for storing and treating runoff. They are also used to attenuate runoff. They have a permanent pool and usually some aquatic vegetation. Wetlands Similar to ponds, but the runoff flows slowly and preferably continuously through aquatic vegetation. They tend to be less suited to use as attenuation structures, depending on the types of plants used. Detention basin Vegetated depressions designed to store water to meet specific attenuation requirements. Soakaways Sub-surface structures that infiltrate runoff. Infiltration trenches As soakaways, but provided as a trench. SR R. 4.0

26 Table 2.5 SUDS techniques - their hydraulic, water quality and amenity capabilities (continued) Amenity Water Quality Hydraulic Performance Key to symbols: Benefits Treatment Primary process Ecology Amenity Aesthetics Nitrification Uptake by plants Precipitation Volatilisation Biodegradation Adsorption Filtration Sedimentation Water Harvesting Infiltration Attenuation Conveyance Secondary process SUDS Technique SUDS Description Infiltration basins Depressions that store and dispose of water via infiltration. Green roofs Vegetated roofs that reduce the volume and rate of runoff and remove pollution. Bioretention areas Vegetated areas similar to swales, but with emphasis on a variety of vegetation including the use of wetland plants. Greater emphasis on treatment. Underground storage Attenuation tanks to reduce stormwater runoff rate Propriety drainage units Range of units: screens, proprietary sedimentation units etc Pipes and manholes Conduits and their accessories normally underground as conveyance measures. SR R. 4.0

27 3. Design Criteria This chapter details the design criteria that are currently applied to drainage and explains why the criteria are required. 3.1 GENERAL Chapter 2 discussed the stormwater runoff implications of urban catchments and the opportunities SUDS provide in mitigating these aspects. To allow a drainage system to be designed, criteria need to be set to ensure that: Flood protection - Adequate flood protection is provided on site - Adequate flood protection is provided as a consequence of the development Water quality treatment - The polluting impact on the receiving water is mitigated Operation and maintenance - The drainage system operates efficiently with minimal risk of failure and minimal need for maintenance Additional criteria need to be considered to address issues of the environment, ecology and amenity. These are detailed in Chapter FLOOD PROTECTION CRITERIA There are four possible different causes of flood risk. These are: 1. Flooding from an adjacent watercourse, 2. Exacerbated flooding of the receiving watercourse downstream, 3. Flooding of the site from the site drainage system, 4. Protection against overland flooding from the site on neighbouring areas, 5. Protection against flooding from overland flows from areas adjacent to the site. The criteria applied to address each of these areas of flood risk are shown in Table 3.1. These criteria can vary depending on the authority or the community involved. 100 year levels of protection are generally used, but these may be raised for particularly vulnerable communities or high value areas. To provide this flood protection for the site and downstream of it entails: provision of adequate conveyance capacity, provision of appropriate temporary and permanent attenuation storage, and provision of infiltration for volume reduction, where possible. SR R. 4.0

28 Table 3.1 Drainage design criteria for flood protection Criteria Design Event Design Objective 1. Protection against flooding from watercourse Maximum water level for the 100 year event Protecting property floor levels. 100yr river level + safety factor (often 500mm). 2. Protection of watercourse from exacerbated flooding 3. Protection against flooding from the drainage system 100 year 6 hour event Difference in runoff volume (preand post development). Retained on site (Long term Storage)*. 30 year event No flooding on site, except where specifically planned and approved. 100 year event No flooding of properties. Long duration events: Floor levels above the maximum flood storage levels on site. High intensity events: Planned flood routing and temporary flood storage on site. 4. Protection against flooding adjacent areas 5. Protection against flooding from hinterland drainage 100 year event Flooding managed on site and not passed to other urban areas. 100 year event Flood routing management of potential flooding from adjacent areas. *Long term Storage is the term given to detained runoff that is disposed of by infiltration to the ground, or if this is not possible, stored temporarily on site and discharged to the water course at very low flow rates. This storage need not come into effect except for extreme events as the intention is to protect rivers at times of extreme river flooding. Long term storage is the name associated with the provision of storage which addresses the additional runoff volume generated by development compared to the predevelopment catchment for the 6 hour 100 year event. This is preferably disposed of by infiltration, but otherwise can be discharged to the river at very low rates of discharge when infiltration is not possible (soil type) or is undesirable (pollution risk etc). The intention is to ensure that this runoff component does not contribute to the flood volume in the river at times of extreme river flows. 3.3 CLIMATE CHANGE CRITERIA Climate change should normally be taken into consideration and this may vary depending on location. Generally agreed rules relating to climate change allowances are as shown in Table 3.2. In addition, it is now recognised that once a development is SR R. 4.0

29 completed, extensions and additional paving often take place. This is sometimes referred to as urban creep. The extent to which this takes place depends on the development type and the density of development. Clear guidance as to an appropriate design horizon for new development has yet to be produced. However between 50 and 100 years is suggested, depending on the implications of climate change. Table 3.2 Current best practice Climate Change and other safety factors Climate change / safety factor aspect Sea level change (depending on coastal location) River flow change Rainfall change Urban creep* Design criteria 4mm/yr - 6mm/yr in sea level 20% increase in peak river flows (2050 horizon) 10% increase in rainfall intensity 10% increase in impermeable area Reference Defra (2000) MAFF (2000), Environment Agency (2003) Defra / Environment Agency (2004) WRc (consultation draft, publication due in 2005) * The application of an allowance for urban creep will be first officially introduced in the 2 nd Edition of Sewers for Scotland (to be published in 2005). 3.4 WATER QUALITY The quality of stormwater runoff is an issue for most rainfall events.. The catchment is regularly washed by every-day rainfall and therefore the majority of the mass of pollutants is washed off by small storms. The generally low flows in the river at these times means that the pollutant concentrations tend to be high compared to large events during periods of flooding when a high degree of dilution is obtained. This polluted quantity of runoff can be treated in various ways, which are outlined in Table 3.3. Table 3.3 Water quality treatment methods and suitable SUDS components Treatment method Filtration through a bed of filter medium, in soils or across a vegetated area Settling of sediment Biological treatment by microorganisms growing on filter media, soil particles, vegetation or suspended in a water body Adsorption of particles on plants or filter media Dilution for the control of persistent pollutants (only applicable for protecting against process failure and minimising impact) Suitable SUDS components Infiltration trenches, Pervious pavements, Filter strips, Filter trenches, Wetlands, Green roofs, Bioretention, Swales Ponds, Wetlands, Detention basins Infiltration trenches, Pervious pavements, Filter strips, Filter trenches, Wetlands, Infiltration basins, Green roofs, Bioretention, Swales, Ponds, Detention basins Infiltration trenches, Pervious pavements, Filter strips, Filter trenches, Wetlands, Infiltration basins, Green roofs, Bioretention, Ponds Swales, Ponds, Wetlands, Detention basins SR R. 4.0

30 The concept of Treatment Volume (V t ) was introduced in the SUDS design manual C521 (Martin 2000a). This design parameter is a volume which could theoretically be applied to any SUDS unit, but in practice is usually associated with the sizing of the permanent pool of a pond to allow the various water quality processes described above to take place. V t is a storage volume which is calculated as being equal to or larger than the runoff volume of 90 percent of all storms occurring in the year. This volume is usually in the region of 12-20mm of rainfall depending on catchment and hydrological characteristics. The empirical formula which was derived is as follows: V t = 9 D (SPR/2 + (1 SPR/2). I) (3.1) where: V t is in m 3 /ha D is the M 5 60 rainfall depth from the Wallingford Procedure map SPR* is the WRAP map SOIL index SPR value obtained from the Wallingford Procedure map, (1981) I is the impervious fraction of the area * It should be noted that the parameter SOIL is usually used here and in other equations to denote the use of the value of SPR of the particular SOIL category. It is felt that the use of SPR is less confusing and this has been used in other equations throughout this document for consistency. Other literature on BMPs relate the treatment requirements to the mean annual event. The mean annual storm is in the region of 3 to 5mm. An example of how to estimate the treatment volume that would capture runoff from 90% of storms occurring in a year is provided in Appendix 2. This treatment volume can be reduced if treatment is provided upstream by other SUDS components for some of the contributing area.. Usually the pond size is approximately 1 times V t, though a definitive position on this matter has yet to be arrived at for the UK. Treatment volume is used to provide treatment based on experience of the benefits obtained. This hydraulic criterion is used as a surrogate for water quality criteria as it is impossible to specifically design stormwater treatment systems to meet particular water quality parameters concentrations. The morphological criteria given in Table 3.4 are aimed at protecting the flow characteristics of the receiving river. Again the aim is to replicate greenfield conditions from the development runoff. Table 3.4 River morphological criteria Hydraulic processes Rate of discharge Volume of discharge Design parameters 1 year event 100 year event All events Application of design criteria 1 year site discharge rate to 1 year greenfield peak rate 100 year site discharge rate to 100 year greenfield peak rate Where possible, through interception and infiltration, prevent direct runoff from the first 5 to 10mm of rainfall. SR R. 4.0

31 3.5 OPERATION AND MAINTENANCE The use of SUDS introduces an element of increased complexity with regard to the site drainage design. The design for self-cleansing velocities in pipes for traditional drainage is relatively simple and well understood. However runoff rates and volumes from swales and pervious pavements, for example, vary considerably depending on the storm event characteristics and antecedent conditions. For ordinary events, flows are much reduced for both aspects. This is not necessarily a problem as the sediment load should be minimal due to the nature of these drainage components. However a system which has a mix of direct runoff (gullies) and SUDS components will need to consider self-cleansing velocities for the elements which may have a significant sediment load. This means that hydraulic models, which can reasonably accurately predict the runoff characteristics of SUDS components, will often be needed to assist in assessing the hydraulic performance of these systems. Table 3.5 suggests pipe velocity criteria that should be applied. The use of 0.3m/s (where it applies) is not aimed at relaxing the gradient requirements of pipe systems, but to take account of the attenuating effects of certain SUDS components and to recognise that 1m/s may be an unrealistic target. Similarly, maximum velocity is applicable to vegetated systems to prevent erosion, and this is also defined in Table 3.5. There are a number of issues which should be considered in designing a drainage system to minimise maintenance effort and ensure trouble-free operation. Design criteria relating to specific SUDS components are not covered here, and reference should be made to relevant SUDS design manuals. Table 3.5 Self-cleansing velocity criteria for pipes serving a SUDS drainage system Criteria Runoff characteristics Design objective Maximum velocity in vegetated systems All runoff < 1.0m/s 1 year event Minimum velocity in Minimal sediment load* >0.3m/s 1 year event pipes Areas served with direct Velocity criteria in Sewers runoff (gully and pipe for Adoption 5 th Edition systems) * where sediment protection is provided by an upstream SUDS component. SR R. 4.0

32 SR R. 4.0

33 4. SUDS Descriptions and Characteristics This chapter provides a description of SUDS components, their advantages and limitations and the important design issues that need to be considered. 4.1 INTRODUCTION There is a wide range of different types of SUDS components. Each type is suited for providing a particular drainage facility. It is important to realise that no one SUDS component is particularly better than another, and that a variety of SUDS is usually needed to ensure an effective drainage system for both water quality and hydraulic runoff control. The following documents should be referred to for a more detailed description of the different SUDS components and their design criteria. Key industry documents Sustainable drainage systems Hydraulic, structural and water quality advice (Wilson, Bray and Cooper, 2004) Source control using constructed pervious surfaces (Pratt, Wilson and Cooper, 2002) RP697: SUDS updated guidance on technical design and construction (to be published Feb 2006 by CIRIA and HR Wallingford) Sustainable Urban Drainage Systems: design manual for Scotland and Northern Ireland (Martin et al, 2000a) Sustainable Urban Drainage Systems: design manual for England and Wales (Martin et al, 2000b) Building Regulations Approved Document H (DTLR, 2000) Interim Code of Practice for Sustainable Drainge Systems (NSWG, 2004) The information needed by a developer on the characteristics of each SUDS component must not only be about their drainage behaviour but also how they can be applied in a modern high-density residential development. The following categories are therefore used to describe the benefits and limitations of each SUDS component and a table of the characteristics of each SUDS component according to the following categories is provided. Hydraulic performance (small and large events) Water quality treatment Amenity (social and ecology) Land use and availability Issues of construction, operation, design and maintenance Brevity has been applied to this section to ensure a clear overview is provided. Chapters 5 and 6 provide a more detailed examination of design layout and construction issues. Chapter 7 provides more information on detailed design. To provide an understanding of the relevance of each category, a brief discussion is provided first. SR R. 4.0

34 4.1.1 Hydraulic performance (small and large events) Runoff from the site must be controlled, otherwise spate runoff into the receiving system or river will occur. Small events have no significance with regard to causing flooding, however their characteristics in terms of runoff from the development are very different from greenfield runoff (when effectively none occurs) and they convey the majority of the pollution from the site into the downstream environment. Large events need to be controlled to minimise spate flood flows and excessive volumes being discharged to the river. Local flooding on the site also needs to be prevented Water quality treatment Water quality treatment is needed to minimise the pollution discharged from the site Amenity (social and ecology) Amenity relates to opportunities the SUDS components provide in terms of aesthetic and other benefits. There is often a degree of conflict between designing for hydraulic benefits, ecological benefits and community benefits Land use and availability Land use, in this context, relates to the space required, the space available and the location where the SUDS component is most likely to be applied on the site Issues of construction, operation and design There are various issues, depending on the SUDS component, which need to be known or are worth emphasising. The coding for each SUDS is very approximate for this category in that construction issues may be a problem where operation is not, or vice versa. It should therefore be used as an indication and more attention should be given to the description of the issues. Chapter 6 provides additional information on construction aspects. 4.2 GENERAL ISSUES FOR ALL SUDS COMPONENTS USING INFILTRATION Approved Document H to the Building Regulations emphasises the importance of considering infiltration techniques in preference to positive drainage where possible. This means that for marginally suitable soils, during very wet periods, some infiltration components will fill and saturate the surrounding ground if there is no overflow. If an overflow is provided to a positive drainage system that is to be adopted, discussions with the appropriate authority on design criteria are recommended. They may require the normal 10 year criteria to be increased to 30 years. However, a pragmatic approach is recommended, recognising that some infiltration is better than none. A communal or continuous infiltration system serving a group of properties is not usually recommended, as failure will result in flooding taking place preferentially at one location, usually to the detriment of one property. Carefully consideration of failure implications can, however, lead to successful communally based systems. Whether communal or individual units, consideration should be given to overland flood paths and the use of overflows where potential failure of infiltration components might result in flooding. This is a particular issue for high-density developments as terraced properties are commonly built, which limits the relief flow paths for flood flows. SR R. 4.0

35 4.3 IN-PROPERTY SUDS (WATER BUTTS, STORAGE FOR DOMESTIC WATER USE) In-property SUDS refers to those systems that store roof runoff either within the building or in a chamber beside it, all of them having an emphasis on domestic or garden use. Other forms of SUDS, which are used within the curtilage, are detailed separately Description Figure 4.1 Water butt Table 4.1 In-property SUDS characteristics for different categories Priority Hydraulic benefits - small events Hydraulic benefits - large events Water Quality Benefits Amenity Benefits Ecological Benefits Land Take Land Availability Rainwater harvesting Benefits Construction Issues High Medium Low None N.B. Water quality benefits are none in that they do not contribute treatment to the surface runoff. They are only used where the risk to groundwater pollution is minimal. Water butts are connected to rainwater pipes and normally store less than 0.5m 3 of water. The size of storage components required for toilet-flushing use is usually 2 to 5m 3 to provide an effective supply for much of the year. The size is related to the hydrology of the area and the water demand placed on the component. It is normally buried adjacent to the house. Treatment of the runoff in the form of filtering is required. Domestic use requires a small electrical pump which raises water to a header tank. SR R. 4.0

36 4.3.2 Hydraulic performance (small and large events) Water butts have no value in providing a reduction in flows downstream from major events, as they cannot be assumed to have any spare storage space to attenuate runoff from a large storm event. They do however have the potential to reduce runoff volume from small events, particularly in summer (as some spare storage capacity can be assumed), which results in less pollution being discharged into the receiving water for a significant proportion of rainfall events. Their use also theoretically reduces the demand on treated water supply, though in dry periods the stored water is exhausted very quickly if used for garden watering. Stormwater stored for domestic use provides a significant level of volume reduction for small events, but has limited impact in providing drainage advantages for large events. The systems are very beneficial in reducing water demand for treated water if the storage volume is large enough Water quality treatment In-property SUDS do not provide any significant benefit in treating runoff discharged from the site. This is because most of the pollution in stormwater runoff is from road surfaces Amenity (social and ecology) In-property SUDS have no ecological benefits. There are health risks associated with domestic use, which need to be taken into account. However, there are potentially significant benefits in reducing water supply demand, especially in water scarce regions Land use and availability Virtually no land is required for these components to be used Issues for construction and operation There are no significant construction implications. Domestic use systems must be robust and easy for homeowners to operate and maintain Issues for design The size of storage for reuse can be calculated based on an assumed demand and a time series rainfall record appropriate to the area. Large storage volumes will result in long retention times which can cause problems of septicity and odour. Provision needs to be made for an overflow to operate. This may need to be piped to a drainage system. Systems can be designed on a communal or individual household basis. Ownership issues, treatment and delivery pipework is more of an issue where communal systems are built. SR R. 4.0

37 4.4 PERVIOUS PAVEMENTS Pervious pavements are now becoming more commonly used in the UK They are frequently used for car parks or pedestrianised areas. They have also been used on local distributor roads, and on home zone roads and in cul-de-sac s. They can be used for heavy goods vehicle parks and in container yards, provided structural issues relating to the loading of the system have been specifically addressed. They allow rainfall to pass between the blocks into the sub-base for temporary storage, and infiltration where ground conditions allow Description Figure 4.2 Pervious pavement Table 4.2 Pervious pavement characteristics for different categories Priority Hydraulic benefits - small events Hydraulic benefits - large events Water Quality Benefits Amenity Benefits Ecological Benefits Land Take Land Availability Rainwater harvesting Benefits Construction Issues High Medium Low None Pervious pavements are often constructed using concrete blocks which are shaped to provide gaps through which rainfall can pass into the sub-base. Other materials include porous asphalt, porous concrete and a range of concrete or plastic products which allow in-filling with grass or granular material. The sub-base can comprise granular material of specific size grading or high voids plastic media. The depth of construction is designed to provide structural support for SR R. 4.0

38 pavement and surface water storage and the granular fill is normally around 350mm, although depths can be designed to be more or less than this value. Treatment can still be achieved in the blocks, bedding layers and geotextile when using plastic sub-base systems Hydraulic performance (small and large events) Pervious pavements have good hydraulic properties with high levels of attenuation when granular fill is used. Runoff volume reduction is very high for small events in dry periods though not particularly good for large events and in wet periods. Pervious pavements can be used to store and attenuate adjacent roof runoff Water quality treatment Water quality from pervious pavements is normally very good with minimal sediment load, and low pollutant concentrations especially if a geotextile is used below the bedding layer for the blocks Amenity (social and ecology) Amenity value is limited, but its use is generally not unattractive, especially as coloured blocks can be used. Vegetation can be planted as the pervious surface allows the runoff to be passed to, and support, the plants Land use and availability As car parking and other hardstandings are essential elements of developments, there is usually no need for additional land to be made available. It is flexible in being able to be used for small or large areas Issues of construction, operation and design The Building Regulations Part H requires infiltration systems to be located 5m from any foundations. However pervious pavements that are serving private car parking areas can be located close to the buildings as they are acting in the same way as if the area had been grassed. If the pervious pavement is receiving additional runoff from roof areas then a lined section for the first metre away from the foundations may be required. Car parks and roads are often built early in the construction process, at least in part, to provide traffic access and to allow storage of construction materials. As the structure is intrinsically intended to be porous, normal practice of using the area for storage and construction traffic is not possible without specific additional measures to protect the pavement porosity (see chapter 6 for more discussion on this aspect). In addition to care being needed in the construction process, the design must take account of the need for minimising any soil or sediment being washed or deposited onto the surface once it is in operation. An advantage of pervious pavements is that the cost of construction is only marginally different to standard pavement construction. As light liquid separators and gully pots are normally not required, this provides additional drainage benefits at very little cost. SR R. 4.0

39 4.5 FILTER TRENCHES Filter trenches are built adjacent to the edge of a road or hardstanding and are used to provide road drainage as an alternative to pipes and gullies. Their principal purpose is to protect the road sub-base from becoming saturated and therefore they are often used in cuttings. Figure 4.3 Filter trench Table 4.3 Filter trench characteristics for different categories Priority Hydraulic benefits - small events Hydraulic benefits - large events Water Quality Benefits Amenity Benefits Ecological Benefits Land Take Land Availability Rainwater harvesting Benefits Construction Issues High Medium Low None Description Filter trenches normally comprise a coarse single size stone, or specific stone mix, wrapped in a geotextile with a perforated pipe at the base of the trench. They are relatively commonly used in the UK. They are included as being a SUDS component due to their hydraulic and water quality advantages Hydraulic performance (small and large events) Filter trenches provide some degree of attenuation and volume reduction, particularly for small events. SR R. 4.0

40 4.5.3 Water quality treatment Water quality of the effluent is much improved compared to a pipe and gully system, although it is less effective than some other SUDS components Amenity (social and ecology) Amenity value is limited, perhaps even being slightly negative Land use and availability The land take is small but requires a strip of land adjacent to roads on which they are used. Filter trenches are not normally appropriate for estate roads and therefore their potential for application is limited in developments. Innovative design to avoid stone scatter by covering the surface has been used elsewhere in the world, but maintenance implications of any solution have to be carefully considered Issues of construction, operation and design Filter trenches, which normally have granular material exposed at the surface, provide a maintenance issue in residential areas and are therefore rarely used. Where they are used under the pavement in conjunction with gully inlets, the risk of blockage in the long term is significant. The upper layer can consist of stone or a bonded mix on a geotextile membrane to avoid problems of stone scatter and blinding by sediment, which can be removed and replaced as necessary to maintain free-draining conditions. They should be sited away from the immediate road edge to avoid potential sideways movement of the road pavement into the trench. SR R. 4.0

41 4.6 FILTER STRIPS Figure 4.4 Filter strip Table 4.4 Filter strip characteristics for different categories Priority Hydraulic benefits - small events Hydraulic benefits - large events Water Quality Benefits Amenity Benefits Ecological Benefits Land Take Land Availability Rainwater harvesting Benefits Construction Issues High Medium Low None Filter strips are vegetated strips of land designed to accept runoff as overland sheet flow from paved surfaces. Their purpose is to provide treatment of runoff by encouraging settlement of fines as flow passes over the surface Description Filter strips are usually several metres wide and encourage filtration of particulate material. They act as a pathway between the runoff and the collection system Hydraulic performance (small and large events) Filter strips provide some degree of attenuation and volume reduction, particularly for small events. SR R. 4.0

42 4.6.3 Water quality treatment Water quality of the stormwater for small events is good depending on the quality and width of the filter strip. During extreme events, and before vegetation is fully established, scouring can occur, and water quality benefits will be minimal until the filter strip is rehabilitated Amenity (social and ecology) Amenity value is positive in providing green open areas. There is limited ecological benefit due to the regular maintenance requirement and the need to provide a smooth plane surface Land use and availability The land take is very significant and virtually impossible to apply in a high density residential area. However there may be some instances where it can be used Issues of construction, operation and design The gently sloping area needs to be carefully constructed and maintained to ensure sheet flow takes place. Erosion can easily occur. SR R. 4.0

43 4.7 SWALES Figure 4.5 Swales Swales are vegetated shallow depressions normally located beside roads. Their purpose is to both convey and treat runoff. Table 4.5 Swale characteristics for different categories Priority Hydraulic benefits - small events Hydraulic benefits - large events Water Quality Benefits Amenity Benefits Ecological Benefits Land Take Land Availability Rainwater harvesting Benefits Construction Issues High Medium Low None Description The term swale covers a range of different drainage designs. The standard or conveyance swale is a shallow vegetated channel of around 400mm deep, up to 5m wide at ground level and 1m wide across the base. This receives sheet flow off roads and hardstandings and conveys it to a point where it is discharged into a pipe network or other structure. The length of the swale is limited primarily by its conveyance capacity and characteristics, as erosion of the base needs to be avoided. Two other forms of swale exist. The first is termed a mini-swale as it need only be one metre wide, 100mm deep and is usually built in short lengths. It receives runoff and stores the water for infiltration, but once it is full, any additional water is discharged into the primary drainage system through an inlet of some kind. SR R. 4.0

44 The third type of swale, a storage or under-drained swale, is similar in size to a standard swale but, instead of having an outfall, it is served by a small perforated pipe located immediately below the swale. This design provides storage of runoff and allows percolation of flow into the pipe and the adjacent ground Hydraulic performance (small and large events) The hydraulic performance of all the swales is good for small events, but only the under-drained swale is likely to be effective in providing good attenuation and volume reduction for large events Water quality treatment Water quality of the stormwater for small events is good for all swale types. The underdrained swale is likely to be the most effective, particularly for larger events when a standard swale may have problems with erosion if gradients are significant. Miniswales will still provide significant water quality benefits in spite of their limited hydraulic capacity Amenity (social and ecology) Amenity value is positive in providing green open areas. There is limited ecological benefit due to the regular maintenance requirement Land use and availability The land take is very significant for standard and under-drained swales. Although very desirable for treating road runoff, the demands of site planning for meeting PPG3 housing density requirements may often preclude their use over much of a site. However their importance in addressing both the volumes of runoff for small events and improving water quality from road runoff, makes these components very desirable in terms of protecting the receiving environment from the most polluting urban surfaces Issues of construction, operation and design Swales are most effective when built with very gentle gradients. In this situation they provide a great deal of storage, good treatment and erosion is unlikely to take place. Although they can be built with longitudinal gradients of 1:20 and steeper, their benefits rapidly diminish as sediment washoff tends to collect at the end of the swale, killing the vegetation and causing an unsightly area. Frequent check dams should be used on any steep sites. There is a need to balance the requirement to have sheet flow runoff into a swale with protecting the structure from vehicles. The use of kerbs results in point flow inputs, which may cause erosion problems. Velocity of flow in a swale needs to be kept low to encourage sedimentation. Road layouts therefore need to use the contours of the site appropriately, unless constructed as a series of nominally level, stepped channels. As swales are normally shallow structures, pipes used to convey flow under a road or driveway into the next swale, need to be designed to be strong enough with the minimal depth of cover available. These pipes offer opportunities for blockage and also erosion. Other solutions such as proprietary channels and gratings offer alternative options which may be appropriate in some situations. SR R. 4.0

45 4.8 PONDS Figure 4.6 Retention pond Table 4.6 Pond characteristics for different categories Priority Hydraulic benefits - small events Hydraulic benefits - large events Water Quality Benefits Amenity Benefits Ecological Benefits Land Take Land Availability Rainwater harvesting Benefits Construction Issues High Medium Low None Ponds are bodies of largely open water into which stormwater runoff is passed for treatment and attenuation. There are two categories and these are referred to as Detention ponds and Retention ponds, the difference being the duration that runoff is retained in the pond, which is a function of the volume of the permanent pool Description A Retention pond is a wet pond where runoff is retained for some days to allow settlement and biological treatment of some pollutants, as well as providing attenuation of the stormwater flows. The permanent pond volume is based on an effective rainfall depth normally in the range of 40 to 60mm. Side slopes should be shallow for safety and aesthetic reasons. An impenetrable planting barrier around the edge of the pond is often included for safety reasons, though there is a counter argument which suggests that safety requires access to be possible. Maximum permanent pool depths are in the region of 1 to 2m, but ponds can often be much shallower. SR R. 4.0

46 A Detention pond is almost identical to a Retention pond, but it has a permanent pool volume based on only 10 to 15mm of effective rainfall. The additional retention period of a Retention pond is generally believed to improve treatment of stormwater runoff. However this is not always the case, particularly where eutrophication is a possibility. The use of a series of ponds is more advantageous than one large pond for hydraulic, water quality, ecological and operational benefits Hydraulic performance (small and large events) Ponds may provide some volume reduction through infiltration and even evaporation. However their main hydraulic benefit is their facility for limiting discharge flow rates. This advantage applies to all sizes of event. Their ability to limit discharges to low flows is really only effective for catchments larger than 3ha. For smaller catchments throttle sizes need to be small and consequently the risk of blockage increases. For small catchments, where permitted, outflow structures are based on the principle of a riser pipe with a limited number of small perforations. Care is needed to provide protection from floating debris of all sizes Water quality treatment The water quality treatment efficiency of ponds is high assuming they are well designed to prevent short-circuiting. Long narrow ponds or ponds in series provide better water quality, more wildlife diversity and higher levels of attenuation than a single pond of the same volume Amenity (social and ecology) Amenity value of ponds is high for well designed ponds, both aesthetically as well as providing good ecological benefits. Safety can be a concern and must be addressed by good design and education Land use and availability The land take of ponds is significant. They require large areas of public open space. In addition, they may be required to be some distance from any property for reasons of safety. However, ponds located closer to properties and in view of residents are safer than ponds that are not overlooked Issues of construction, operation and design Ponds can be very useful in providing protection to the environment during construction by trapping the high loads of sediment in stormwater runoff. If they are used for this purpose, facilities to remove sediment need to be incorporated into the design. Rehabilitation of the pond (removal and disposal of sediment and other detritus) will need to be carried out prior to adoption by the relevant authority. The design of the pond should specifically cater for future maintenance requirements. This includes the potential impact on the receiving environment in carrying out sediment removal or vegetation management. It should also consider the types of equipment that are likely to be used (reach distance from the bank) and also the SR R. 4.0

47 operation and maintenance access and space needed for the equipment and supporting vehicles to operate. It may be appropriate to provide areas for deposition and consolidation of the excavated material. If the ponds are intended to provide infiltration capability around the perimeter, the construction process must avoid damaging the ground. The hydraulic design of a pond outlet needs to consider the head-discharge relationship including any downstream backwater effects where the receiving water level can rise sufficiently to affect the outflow rate. Ground water levels (in winter) need to be established to determine the depth to which ponds or basins can be excavated, or choosing the permanent pool water level. Groundwater should be at least 1m below the base of the pond unless it is lined. Multiple ponds in series are preferable to a single large pond for maintenance reasons as the need for specialist (large) equipment / plant required for maintenance is avoided. The use of a liner should be considered where water levels need to be maintained or the groundwater requires protection. SR R. 4.0

48 4.9 LINEAR PONDS Figure 4.7 Linear pond Linear ponds (which are similar in many ways to the simple ditch) are similar to retention ponds but are used as an alternative to swales in serving runoff from roadways. Ponds are designed specifically for hydraulic criteria whereas linear ponds are constrained by the space and opportunity to use them. Table 4.7 Linear pond characteristics for different categories Priority Hydraulic benefits - small events Hydraulic benefits - large events Water Quality Benefits Amenity Benefits Ecological Benefits Land Take Land Availability Rainwater harvesting benefits Construction Issues High Medium Low None Description A linear pond is a vegetated channel of around 1.0m or more deep with relatively steep side slopes. A small depth of permanent water is retained in the channel at all times and wetland type vegetation will grow in this region. The linear pond receives gully or sheet flow runoff from roads and conveys it down to a point where it is discharged into a pipe network or other structure Hydraulic performance (small and large events) The hydraulic performance of linear ponds is good, providing conveyance, attenuation and storage for runoff. SR R. 4.0

49 4.9.3 Water quality treatment Water quality treatment of the stormwater is good for small events, particularly due to the presence of wetland type vegetation in the base of the channel Amenity (social and ecology) Amenity value is positive due to the provision of terrestrial and aquatic habitat. Safety may be an issue. They can also provide physical security in a less intrusive way than a fence Land use and availability The land take for linear ponds is slightly less than that required for swales and they can be used alongside roadways to treat runoff Issues for construction, operation and design Linear ponds are very useful in providing drainage facilities for areas with very flat gradients. In addition issues of inlet design for runoff and outlet structures are much easier to provide than for swales due to extra depth of the component. Maintenance is much reduced compared to swales with vegetation needing attention only on an annual basis. SR R. 4.0

50 4.10 WETLANDS Figure 4.8 Wetland Table 4.8 Wetland characteristics for different categories Priority Hydraulic benefits - small events Hydraulic benefits - large events Water Quality Benefits Amenity Benefits Ecological Benefits Land Take Land Availability Rainwater harvesting Benefits Construction Issues High Medium Low None Wetlands are similar to ponds but are dominated by aquatic vegetation. Their design tends to be more complex. Their main purpose is to maximise water quality treatment Description The main difference between ponds and wetlands, apart from the emphasis on vegetation, is the need for continuous flow into the wetland to ensure healthy plants. This is currently a particular problem for ownership as land drainage responsibilities are separated from those of providing urban drainage services Hydraulic performance (small and large events) Although wetlands potentially provide identical opportunities to control runoff as ponds, in practice they may not provide the same degree of hydraulic benefit. This is because plant management requires a more controlled water level range. However, many wetland plants do not mind periodic inundation for up to 48 hours and therefore substantial attenuation can be achieved. A good knowledge of appropriate plant species and their tolerance to water level change is needed if the wetland is used as the main SR R. 4.0

51 attenuation system. In addition, as the plants require a constant base flow, the stormwater inflow should be supported by land drainage or other inflow Water quality treatment The water quality of the effluent can be better than for attenuation ponds, but this is dependent on effective management of the system. Plant die off in winter can pose problems of nutrient release. Drying out of the wetland must be avoided to maintain good treatment. Careful management is therefore needed to ensure a high quality of effluent Amenity (social and ecology) The amenity value of wetlands is less than ponds as open water is considered to be more attractive. Although wetlands are ecologically beneficial, this depends to a large extent on the design, plant type and diversity. Variation in depth, open water and vegetation cover maximises the opportunity for bio-diversity Land use and availability The land take of wetlands is significant. They need to be located in large areas of public open space Issues of construction, operation and design Wetlands have similar construction and operational needs to ponds. However there is generally a need for greater technical detail with regards to the provision of appropriate growing media for the aquatic plants, depth of flow and consideration of water level range. SR R. 4.0

52 4.11 DETENTION BASINS Figure 4.9 Detention basin Detention basins are dry depressions aimed primarily at providing attenuation storage, but they also to provide reasonable treatment of stormwater runoff Description Detention basins are normally grassed depressions or areas surrounded by embankments that store stormwater temporarily to limit the discharge rate of stormwater passing downstream. Table 4.9 Detention basin characteristics for different categories Priority Hydraulic benefits - small events Hydraulic benefits - large events Water Quality Benefits Amenity Benefits Ecological Benefits Land Take Land Availability Reuse Benefits Construction Issues High Medium Low None Hydraulic performance (small and large events) Detention basins have an advantage over ponds in providing stormwater attenuation and volume reduction in that they can be provided in a smaller space. The maximum depth of attenuation storage is usually greater and initial runoff losses are higher. Similar to ponds, the level of attenuation is limited by the minimum throttle size, depending on the stipulations of the adopting authority. SR R. 4.0

53 Water quality treatment The water quality of the stormwater is generally not as good as for ponds, but it is generally quite effective, subject to shape. A problem particular to basins is the difficulty of avoiding silt being re-mobilised and washed out by a large storm and possibly causing a pollution incident. Although the concept of detention time to achieve water quality improvements can be applied to basins, extended detention requires either very small orifice sizes or very large areas to be drained. In practice where water quality is an important requirement, a pond is usually used Amenity (social and ecology) The amenity value of basins tends to be limited, though they still provide green open spaces. However, the debris left in the bottom of the basin tends to be unsightly, making their use more suited to less visible locations. Vegetation growth tends to look unattractive unless well managed. Ecological benefits are provided, although these are generally less than for wetlands and ponds Land use and availability The land take of basins is relatively small compared to ponds and wetlands. Because they can be effective in smaller areas, they can be located around a site, although being less attractive than ponds, they are not normally placed in highly visible areas Issues of construction, operation and design Basins have no particular construction issues. As with ponds, they provide an opportunity to manage site pollution, although rehabilitation is likely to need carrying out once the site is established. Maintenance is required to keep outlets free of blockages and keep negative aesthetic elements to a minimum. SR R. 4.0

54 4.12 SOAKAWAYS AND INFILTRATION TRENCHES Figure 4.10 Soakaway Table 4.10 Soakaway and infiltration trench characteristics for different categories Priority Hydraulic benefits - small events Hydraulic benefits - large events Water Quality Benefits Amenity Benefits Ecological Benefits Land Take Land Availability Rainwater harvesting Benefits Construction Issues High Medium Low None N.B. Water quality benefits are none in that they do not contribute treatment to the surface runoff. They are only used where the risk to groundwater pollution is minimal. A soakaway is a subsurface structure into which surface water, normally from roofs, is conveyed. An infiltration trench is identical except that soakaways tend to be round and deep, whereas infiltration trenches are linear structures and are shallower. They are both designed to dispose of surface water by infiltration Description A soakaway can be constructed using perforated concrete rings with a granular surround and base, however most nowadays are constructed using plastic units. This provides a combination of large storage volume and high surface area for infiltration. Soakaways for small dwellings comprise a void of around one cubic metre lined with a geotextile and filled with granular material. SR R. 4.0

55 Hydraulic performance (small and large events) Soakaways are normally designed to provide at least a 10 year return period level of service. In practice the conservative design assumptions (such as 100% runoff and a safety factor applied to the infiltration rate) mean that a higher level of service is commonly provided. Soakaways provide significant drainage benefits for volume reduction of runoff for both small and large events. Infiltration trenches have two additional benefits over soakaways. The first is that the hydraulic requirements often result in a smaller volume of excavation due to the volume to surface area ratio providing a higher level of infiltration capacity. The second advantage is that where the groundwater level may preclude the use of a soakaway, an infiltration trench might still be feasible. To protect the component it is useful to provide a trap to prevent detritus and sediment passing into the soakaway resulting in long term clogging and eventual blockage Water quality treatment The water quality of roof runoff into the ground is usually not a concern for polluting groundwater. Use of soakaways for other paved surfaces needs to consider issues of the sensitivity of the groundwater (groundwater zones), and the characteristics of the runoff Amenity (social and ecology) There is no amenity or ecological benefit from the use of soakaways. However, recharging of groundwater is an important aspect and meets the Building Regulations requirements to consider the use of infiltration before other drainage options. In addition, there are financial benefits for householders, as the surface water element of the drainage charge is not made to properties drained by soakaways Land use The land take of soakways is small. However Approved Document H to the Building Regulations requires soakaways to be located 5m from foundations. High density developments may have some difficulty in complying with this stipulation. It is possible that infiltration trenches, with their reduced depths, could have this distance requirement relaxed. Some of the reasons for this rule include: Certain soils are susceptible to shrinkage through the wetting and drying process. Softening of soils can occur when water is present. There is a loss of fines caused by wash out and consolidation of loose soils by water percolation Issues of construction, operation and design Soakaways and infiltration trenches have no particular construction issues other than constraints related to soil types and groundwater. Design of drainage systems should always first consider using infiltration for roof drainage in compliance with Building Regulations Part H. SR R. 4.0

56 4.13 INFILTRATION BASINS Figure 4.11 Infiltration basin Table 4.11 Infiltration basin characteristics for different categories Priority Hydraulic benefits - small events Hydraulic benefits - large events Water Quality Benefits Amenity Benefits Ecological Benefits Land Take Land Availability Rainwater harvesting Benefits Construction Issues High Medium Low None N.B. Water quality benefits are none in that they do not contribute to the surface runoff. They are only used where the risk to groundwater pollution is minimal. An infiltration basin is a dry basin, in an area that has highly permeable soils. The base might be stripped or vegetated. Occasionally, a constructed component using sand and gravels is placed over the soil Description An infiltration basin aims to dispose of runoff from most events by infiltration, but large events are catered for by way of additional storage and an overflow. They usually serve runoff from all types of surfaces and therefore sediment loads and potential risk of blinding need to be considered in both the design and the management of the system. The base of the component may require scarifying on an occasional basis. The use of granular material and sands, with geotextiles, provides an alternative approach to maintain the infiltration capacity of the system. SR R. 4.0

57 Hydraulic performance (small and large events) The long term performance of these components is dependent on how well the maintenance regime manages sediments. These components, if operating correctly, are highly effective in reducing runoff volumes for both small and medium-sized events Water quality treatment Use of infiltration basins for road and other paved area runoff needs to consider issues of the sensitivity of the groundwater and the characteristics of the runoff. Provision of light liquid separators should be considered for areas likely to be contaminated by oils and fuel Amenity (social and ecology) The amenity or ecological benefits of these components is related to their design. There is a significant risk that the aesthetic effect will be negated due to accumulated washoff of sediments and general debris. Regular maintenance is likely to be needed Land use and availability The land take of infiltration basins is usually high, depending on the design criteria and the soil porosity. By definition the characteristics of these components is for the provision of large flat areas to achieve disposal of the runoff Issues of construction, operation and design The area to be used for infiltration must be carefully protected from construction activity and traffic. In addition, great care must be exercised in minimising sediment runoff into the basin. It is suggested that the best way to ensure these requirements is to build the basin very late in the programme of the development, and provide alternative means of drainage for the period of construction. Covering the surface with a geotextile to trap sediment arising from construction activities may be appropriate. SR R. 4.0

58 4.14 GREEN ROOFS Figure 4.12 Green roof Table 4.12 Green roofs characteristics for different categories Priority Hydraulic benefits - small events Hydraulic benefits - large events Water Quality Benefits Amenity Benefits Ecological Benefits Land Take Land Availability Rainwater harvesting Benefits Construction Issues High Medium Low None Green roofs are a multi-layer system that covers the roof of a building with vegetation over a drainage layer (Wilson et al 2004). They are used for a range of reasons (insulation for noise, heat and cold) and not just drainage where it provides some attenuation and volume reduction Description Green roofs can be used on both flat and pitched roofs, although the roof pitch is normally less steep than a traditional domestic roof. The construction involves light weight growing media and a water proof liner. The plants normally used are Sedums which are extremely robust in coping with both drought and a big temperature range. Green roofs, though still rare in UK, are becoming more common in the UK, particularly in London and they are a key component of SUDS applications in some other countries. As roofs comprise a large proportion of the urban hard surface, green roofs are likely to play an increasingly important part in providing a more sustainable urban environment. SR R. 4.0

59 There are two main types of green roof (British Council for Offices, 2003). Extensive roofs where the entire roof area is covered in low-growing, low maintenance plants. Intensive roofs these are landscaped highly managed environments that can even include trees. Intensive roofs tend to be used in city centre areas where land is at a premium and high value roof gardens are built Hydraulic performance (small and large events) Green roofs are effective in providing both attenuation and reduction of runoff for small events. These advantages are reduced for larger events in terms of hydraulic reduction and attenuation. Where runoff passes to soakaways, an interception chamber is needed to trap vegetative matter and sediments. Where reuse is considered, green roofs are at a disadvantage compared to traditional roofs as runoff volume is reduced and the water quality requires more attention Water quality treatment The water quality from green roofs is good in the majority of roofs where high mineral content substrate is used because the runoff is filtered. The higher the mineral content in the substrates used, the better the water quality Amenity (social and ecology) For sedum only roofs there is little amenity or ecological value as they provide a limited environment for flora and fauna. Green roofs are generally positive aesthetically, although as they are yet to be used widely in UK, their aesthetic value has yet to be established. Intensive type roofs are often designed to be accessible and may also include water features and storage of rainwater for irrigation. These types of roofs have a higher amenity value but have high maintenance costs Land use and availability Green roofs require no land take Issues of construction, operation and design The implication for construction of using green roofs is minimal, although expert organisations should be involved. The season will influence requirements to ensure the plants get established properly. The time needed to build a green roof is slightly longer than for a standard roof and provision should be made for this. Green roofs may not be suitable for collection and reuse of stormwater as bacteria and organic load can be relatively high. The reduced volume of runoff for small events means that larger storage components would be required to provide the same level of service in terms of reuse. Calculation of the runoff yield from the many small rainfall events that take place is much more uncertain. SR R. 4.0

60 4.15 BIO-RETENTION Figure 4.13 Bio-retention area Table 4.13 Bio-retention characteristics for different categories Priority Hydraulic benefits - small events Hydraulic benefits - large events Water Quality Benefits Amenity Benefits Ecological Benefits Land Take Land Availability Rainwater harvesting Benefits Construction Issues High Medium Low None Bio-retention areas are effectively small storage basins with forms of vegetation other than grass. They are often provided in the form of an attractive border adjacent to the paved area that they serve Description Where swales generally use grass as the vegetation cover, bio-retention areas use a range of appropriate vegetation, often wetland type plants, for treatment of runoff. They therefore can range from flower borders, through to marshy grasses. Conveyance is not normally an aspect of bio-retention, so an overflow structure is usually provided to address larger volumes of runoff Hydraulic performance (small and large events) Bio-retention sites tend to provide effective storage for small events and a degree of attenuation for large events. Their hydraulic effectiveness depends very much on their design. SR R. 4.0

61 Water quality treatment The water quality from bio-retention areas can often be better than for swales. However where it is used as a vegetated border, horticultural practices will affect the water quality to some degree Amenity (social and ecology) The amenity value is potentially high, depending on its design and management. The ecological benefits will also vary for the same reason Land use and availability Bio-retention areas are generally limited to relatively flat areas. However, they can be any size to suite the environment. As they are relatively flat depressions, the land take is large relative to the hydraulic benefits provided Issues of construction, operation and design There are minimal construction implications for use of bio-retention areas. However care should be made not to compact the ground during construction. Mulches should not be used, as they reduce available storage and can block overflows and downstream systems. SR R. 4.0

62 4.16 SUMMARY OF SUDS PERFORMANCE CHARACTERISTICS The performance characteristics of the different SUDS components are summarised in Table 4.14 which brings together the performance characteristics described in each section of this chapter. Table 4.14 Summary table of performance characteristics for SUDS components Priority Hydraulic benefits - small events Hydraulic benefits - large events Water Quality Benefits Amenity Benefits Ecological Benefits Land Take Land Availability Rainwater harvesting Benefits Construction & Operation Water butt/storage Pervious pavement N/A Filter trench Filter strip Swales Ponds Linear ponds Wetlands Detention basins Soakaways N/A Infiltration trenches Infiltration basins N/A N/A Green roofs Bio-retention Key: Worst Useful Best Water quality benefits are not included for infiltration units as they are not part of the surface runoff system. It is presumed that they are only used where the potential for groundwater pollution is minimal. SR R. 4.0

63 5. Application of SUDS This chapter looks at methods of applying SUDS in high-density developments and the locations in which SUDS can be used. It also draws attention to issues that need to be considered when using each type of SUDS component. SUDS components can replace or complement traditional pipe and gully drainage systems on a development site. The type of SUDS system that is appropriate in any location depends on the catchment it serves, the soil type and the space available. The following section looks at the opportunity and suitability for SUDS use and their benefits. These benefits are the direct benefits in terms of water management. Secondary amenity and other benefits, such as soundproofing and insulation provided by green roofs, are not detailed. 5.1 DEVELOPMENT LAND USE Due to pressure for new development and the demand for housing, particularly in the south of England, guidance note PPG3 was issued by the DTLR (DTLR, 2000) which emphasises the need for the footprint of developments to be minimised. PPG3 suggests a housing density of between 30 to 50 dwellings per hectare (net), which is higher than traditional residential developments which are in the range of 15 to 20. It also emphasises the need for a range of housing types to be built within the development and requires a reduction in the provision of car parking spaces (see Appendix 6). The implications of this policy that affect the use of SUDS are therefore: A high proportion of impervious surface area within a site, Reduced land availability for surface drainage structures, Smaller gardens with reduced opportunity to use soakaway techniques, Communal/ mews parking areas, An emphasis on planned communal public open space. To meet all the planning requirements results in housing which is either terraced, and fronts onto the road, or in housing built around a courtyard. This is illustrated in Figure 5.6. The layout of the housing dictates, to a large extent, the types of SUDS components that can be used. Brownfield development is preferred to building on greenfield areas. Housing density in these areas is often even greater than 50 house per hectare, with little or no green space provision. This difference also links through to a difference in approach to SUDS use in these situations. The choice of infiltration based SUDS on previously developed land is sometimes constrained due to previous contamination of the soil. Infiltration can be used on contaminated sites provided the contaminants are not mobilised. For example, soakaways can be located below the contaminated soil. The different types of land area available and the proportion of each type of land use typically found in new residential developments is given in Figure 5.1. The figure also shows the SUDS components that might be considered in each land use category. SR R. 4.0

64 Approximately 60% of the area of a new development site is made up of impermeable surfaces and 40% remain green space (20% private and 20% public). The degree of impermeability can be significantly greater, particularly in commercial and industrial areas. Figure 5.1 Typical division of land use for new developments and opportunities for SUDS use 5.2 INTEGRATED DESIGN It is important to recognise that, although the hydraulic criteria currently imposed as a discharge consent (typically as part of a planning condition) can usually be achieved by a single pond at the downstream point of the drainage system, a more cost effective and efficient system, both in terms of water quality and hydraulics, can be achieved using a mix of SUDS components placed in series and distributed through the development. A large pond at the low point in the site can be very attractive. However it means that much of the public open space must be located there, thus constraining the master-plan design for the site. In addition, the outfall from the low point from the site may be hydraulically affected by the level of the receiving drainage system or stream. Other issues such as sediment deposition and removal from a downstream pond can be minimised with the use of other appropriate SUDS components upstream of it. Theoretically there is a hierarchy of SUDS components in terms of hydraulic efficiency (both by volume reduction or attenuation) (see Appendix 7, Laughlan, 2004). In practice the constraints on type of SUDS that can be used (due to soil type, site location etc) means that there is a limited choice of components for any particular part of the development. SR R. 4.0

65 As an initial guide, the priority for drainage design should be to maximise the use of infiltration systems (and water harvesting if appropriate) across the site and to make the most of opportunities to use pervious pavements (as they take no extra land) where the use of this system can be allowed. The resulting demands on attenuation and flood storage will then be much reduced. The use of an attenuation pond at the outfall should preferably be a series of small ponds rather than a single pond for water quality, ecology and maintenance reasons. However it is recognised that a single large pond will generally be considered as being of greater aesthetic value. This drainage approach allows more flexibility with regard to the layout of the site and the location of public open space. Figure 5.2 provides a flow chart of decision guidance which will assist in highlighting the use of appropriate SUDS for a development. Figures 5.4 and 5.5 provide an indication of the opportunity to use SUDS components in a high density development. The emphasis on communal parking can be seen and the opportunity this provides to use pervious pavements. Where pervious pavements can be used for footways and home zone roads, the opportunity significantly increases. The figures also show that the possibilities for the use of swales are limited to distributor roads due to the frontage of buildings being so close to their access roads. Much of the green area is provided as private gardens behind the dwellings. If soil characteristics are suitable, infiltration techniques should be used to make use of these areas. 5.3 IN-CURTILAGE AREAS In-curtilage, or private land within a development makes up around 50% of the overall area of the development and therefore suitable SUDS should be considered for use in these areas if stormwater runoff is to be dealt with effectively. Of this area, nearly half is taken up by the footprint (roofs) of the properties. The selection of appropriate SUDS therefore needs to consider not only issues of hydraulic performance but also the risks related to private SUDS ownership, particularly the risks of change of use and minimal maintenance Applicable In-curtilage SUDS components The opportunity for the use of SUDS within a private property is related to three surface categories found within the curtilage boundary. These are: Roofs Gardens Parking areas Roof areas The runoff from the roof area can be harvested using SUDS components such as gutter storage, waterbutts, chambers and green roofs. As groundwater issues and the use of infiltration are very important, Figure 5.3 is provided to assist with decisions on the use of infiltration components. This figure is from CIRIA Report C521 (CIRIA, 2000) SR R. 4.0

66 Direct Runoff Water Supply Soil Outfall Size of Development Use of SUDS in High Density Developments Site Characteristics Consequence for ponds/basins Consequence for other SUDS Is site less than 3 ha? Single area of Public Open Space less than 5% of catchment? Y Y Use of pond may not be possible due to minimum allowable size of control orifice Use of pond to serve the whole area may not be possible due to insufficient area. Basins may be more appropriate Emphasis on other forms of attenuation, such as pervious pavements, is needed Is groundwater category zone 1 or 2? Y Use of pond may require a liner Emphasis on roof water infiltration is required (if approved). Other SUDS units may require a liner to prevent infiltration Is groundwater level high? Y Pond liner will probably need to be used; prevent floatation. Permanent pool water level should be higher than maximum groundwater level Use of infiltration may be limited if groundwater level is within 1.5m of base of unit Is pond outfall a free discharge? N Backwater influence on discharge to be evaluated for pond hydraulic performance Is soil permeable? Y Pond liner should be used unless the water level of the pond can be allowed to drop below the outfall orifice (aesthetics) Maximise the use of SUDS units with infiltration techniques Is soil contaminated? Y Ponds will require a liner All SUDS units used will probably need to be lined Is site in area of high of water supply demand to available resource? Y Emphasis on re-use should be given Is there significant area of direct runoff from roads Y Pond requires forebay for sediment deposition Use of swales, filter trenches or linear ponds is advised Use unlined ponds in series to treat stormwater Groundwater Figure 5.2 Decision guidance for use of appropriate SUDS components SR R. 4.0

67 Courtesy of CIRIA Figure 5.3 The use of SUDS and infiltration SR R. 4.0

68 possible location of swales along roadways possible location of pervious pavements Original schematic courtesy of WSP Group Figure 5.4 Possible location of SUDS components in a new development Gardens Infiltration systems (soakaways, infiltration trenches) can be placed in garden areas to deal with the runoff from the roof surface. Approved Document H to the Building Regulations recommends that infiltration systems should not be located within 5m of the foundation of the property. This is particularly relevant where deep soakaways are used and in areas with problematic soils. In these situations the opportunity to use infiltration in PPG3 compliant developments may be limited. However where soils are less of a problem and with shallow based systems, such as infiltration trenches, this distance can often be reduced. Mini swales, serving the local road, can be located in private gardens where there is sufficient space between the road and the property. The design of many high density developments provides very little space between the house and the road pavement, therefore there is often no option for using this SUDS component (see Figure 5.6). Consideration of the risks of change of use, fertiliser and weed killer application, also need to be made. Driveway (car parking) Although there is emphasis on reducing car parking provision, there is often some offroad car-parking within private properties. Permeable pavements can be used and roof runoff can be passed into the sub-base. The same constraints on distance from the house for using infiltration theoretically apply as discussed earlier due to requirements in the Building Regulations Part H. However, as the depth of a pervious pavement is only in the region of 350mm, some authorities allow infiltration from pervious pavements as close as 1m to the foundations or right up to the foundations if the pervious pavement is only taking direct rainfall. There is rarely a need to provide an overflow due to the high storage capacity of such components assuming overground flow which will not affect the property. SR R. 4.0

69 possible location of swales receiving stream possible location of pervious pavements Original schematic Courtesy of Roger Evans Associates Figure 5.5 Possible location of SUDS components in a new development Lined pavements with a low level outflow to positive drainage, where they are considered necessary, are still useful as the discharge is heavily attenuated in the granular media. The risk of car-parking areas being modified or sealed is relatively high. Methods of ensuring such changes do not take place need to be considered where pervious pavements are used in private properties. SR R. 4.0

70 Figure 5.6 House frontage directly onto the road 5.4 ROADWAYS Roadways make up approximately 30% of the land area within a new residential development. Design Bulletin 32 (Department of the Environment and Department of Transport, 1992) defines the different roadways as described below. In larger developments (greater than 20 ha) the percentage of space is relatively evenly divided between these different types of roadway, with each roadway type making up around 10% of the overall site area. In smaller developments, particularly brownfield sites, the predominant roadway type is access roads. District distributors These roads distribute traffic between the residential, industrial and principal business districts of a town and form the link between the primary road network and the roads within residential areas. District distributor roads are not a road type found within a residential development. Only local distributor roads, major residential and minor residential access roads are of relevance. Local distributors These roads distribute traffic within districts. In residential areas, they form the link between district distributors and residential roads. Residential access roads These roads link dwellings and their associated parking areas to distributor roads. Home Zone roads Included as a subset of minor residential access roads are home zone roads. In the UK the term Home Zone is used for a street where people and vehicles share the whole of the road space safely, and on equal terms; and where quality of life takes SR R. 4.0

71 precedence over ease of traffic movement. (Home Zone design guidelines, Institute of Highway Incorporated Engineers, 2002) Home zone roads have low traffic speeds (10 to 20 mph) and volumes (recommended traffic flow of no more than 100 vehicles in the afternoon peak). In addition, residential buildings in a home zone should have an active front to the street, which means that front gardens are absent or minimal in these areas. Communal parking areas The provision of parking areas can take a range of forms such as on-street parking, offstreet parking, or communal parking areas. The allocation of space for parking varies considerably between different developments, but there is an advised maximum amount of 1.5 parking spaces per property. In new developments where high density living is promoted, communal parking spaces are often provided as the principal parking provision Applicable SUDS components for roads Permeable pavements Roadways, cycleways, footways and parking areas provide opportunity for using permeable pavements. At present their use is normally limited to car parking, but there are instances of their application in home zone road. This means that permeable paving could be applied to at least 10% of the total surface area within a development site, and possibly significantly more if their use on more heavily trafficked roads becomes more accepted. Figure 5.7 A Home Zone road with pervious pavement SR R. 4.0

72 Pervious pavements do not take up any additional space compared to a traditional road surface and therefore do not have any additional land-take. They are particularly useful in home zone areas where traditional kerb and gully drainage systems detract from the integration of the roadway and pedestrian surfaces (Figure 5.7). Swales and linear ponds Other types of SUDS that could also be used to serve road runoff include filter trenches, swales and linear ponds. These systems are located beside the roadway. Swales or linear ponds can also be located in landscaping areas in car parks and other similar locations. These SUDS systems can only be used where the house frontage is set back from the road. 5.5 PUBLIC OPEN SPACE Up to 20% of a residential development site is provided as public open space. The Town and Country Planning Act 1990 defines open space as land provided as a public garden or used for the purposes of public recreation. More recent government policy statements (DTLR, 2002 PPG17) provide a broader definition such that open space can include incidental open space and landscaping as well as open spaces serving a wider area, and significant landscape buffer strips. PPG 17 includes the following definitions that are relevant to open spaces in new developments that may be of public value: natural and semi-natural urban green spaces - including woodlands, urban forestry, scrub, grasslands (e.g. downlands, commons and meadows) wetlands, open and running water, wastelands and derelict open land and rock areas (e.g. cliffs, quarries and pits); green corridors - including river and canal banks, cycleways, and rights of way; outdoor sports facilities (with natural or artificial surfaces, either publicly or privately owned) - including tennis courts, bowling greens, sports pitches, golf courses, athletics tracks, school and other institutions playing fields, and other outdoor sports areas; amenity green space (most commonly, but not exclusively in housing areas) - including informal recreation spaces, green spaces in and around housing, domestic gardens and village greens; provision for children and teenagers - including play areas, skateboard parks, outdoor basketball hoops, and other more informal areas (e.g. 'hanging out' areas, teenage shelters); Public open space is often made up of a number of small open spaces amongst other land uses, though large developments do usually have one or two large areas, often specifically aimed at recreational use. In Scotland the term open space covers greenspace consisting of any vegetated land or structure, water or geological feature in an urban area and civic space consisting of squares, market places and other paved or hard landscaped areas with a civic function (Scottish Executive, 2003). There are, however, no definitive rules for the provision of different types of public open space or how SUDS may be incorporated into public open space areas. SR R. 4.0

73 Figure 5.8 Communal parking areas Figure 5.9 Linear pond In England, PPG17 (refer Table 5.1) does mentions that Local Authorities should recognise that most areas of open space can perform multiple functions and they should take this into account when applying the open space policies. SUDS are not directly referred to. SR R. 4.0

74 However, the open space functions, as defined in Table 5.1, do indicate that SUDS are an appropriate use of open space areas. For example, ponds and wetlands do provide habitats for flora and fauna, if well designed they are a visual amenity and as they are green spaces they can improve the quality of the urban environment. The companion document to PPG 17 (ODPM) notes that different types of open space may also include areas of running or static water such as ponds, fountains, rivers, canals, lakes and reservoirs. Water can make a major contribution to the quality and nature of the greenspace and be an important component of the urban drainage system or vitally important for recreation and biodiversity contribution. In Scotland the Planning Policy Guidance NPPG 11 recommends that for new general purpose housing 1.62 hectares of land per 1000 people (or 0.4 hectares per 100 houses) should be made available as open space. This should comprise 0.81 hectares of amenity open space and 0.81 hectares of recreational open space. The detail of how this open space is allocated is left to the Local Authority. This distinction can be considered as being passive and active respectively; terms which are now becoming more prevalent in defining green space. Table 5.1 Functions of public open space in PPG 17 (ODPM, 2002). Function Strategic functions Urban quality Promoting health and well-being Havens and habitats for flora and fauna As a community resource As a visual amenity Definition Defining and separating urban areas; better linking of town and country; and providing for recreational needs over a wide area. Helping to support regeneration and improving quality of life for communities by providing visually green spaces close to where people live. Providing opportunities to people of all ages for informal recreation, or to walk, cycle or ride within parks and open space or along paths, bridleways and canal banks. Allotments may provide physical exercise and other health benefits. Sites may also have potential to be corridors or stepping stones from one habitat to another and may contribute towards achieving objectives set out in local biodiversity action plans. As a place for congregating and for holding community events, religious festivals, fĕtes and travelling fairs. Even without public access, people enjoy having open space near to them to provide an outlook, variety in the urban scene, or as a positive element in the landscape. Planning documents in England and Scotland also make reference to various other open space considerations. For instance, the National Playing Fields Association (NPFA, 2001) recommends that land is set aside for outdoor play, games, sports and other physical recreation. They suggest a minimum standard for outdoor playing space of 2.4 hectares for 100 houses, comprising 1.6 hectares for outdoor sport and 0.8 hectares for children s play. PAN 65 (Scottish Executive, 2003) provides an example of the use of SUDS in public open space. In Dunfermline open space was provided as a series of attractive ponds. The ponds were an important component of the drainage system, provided pollution SR R. 4.0

75 control and ecological benefits as well as being a significant amenity feature and forming a major part of the landscape structure of the area. Planning guidance for Wales and Northern Ireland do not make specific reference to the use of SUDS in public open space areas Applicable SUDS components in public open space Because of the space requirements of some SUDS components open space areas are particularly suitable for their use. However, in most instances the Local Authority must be satisfied that the SUDS component is a suitable use of the public open space and of benefit to the community. Ponds and Basins Ponds generally need relatively large areas of open space. A pond that serves all the runoff from a development site will require between 2% to 4% of the overall catchment area (which is approximately 10% to 20% of all the available open space area). Basins take around half the land required by ponds. These SUDS components can therefore only be located in developments where there is appropriate provision of public open space. Depending on the adopting authority, the type of open space used is usually limited to passive or amenity provision, to limit the maintenance element to the requirements of the SUDS as a drainage system rather than in providing a community play area. It is important to note that the size of these components can be significantly reduced where other SUDS components (particularly storage and infiltration systems) are used further up the system. Infiltration basins are not often used due to the requirement of highly porous soils and the maintenance implications of preventing blinding of the component with fine sediments. Such SUDS components often serve the stormwater drainage of large areas of the development. Due to controls on their outflow rates, they are usually used to limit the discharge from a site before it enters any downstream drainage system or watercourse. Wetlands Wetlands are designed primarily as treatment systems as vegetation can suffer from the varying water levels which are a feature of hydraulic attenuation structures. Therefore their use in residential areas is less common due to land take as well as the operation and maintenance implications. Swales, Linear ponds and Filter Trenches Other SUDS components such as swales, linear ponds and infiltration trenches, which serve runoff from roadways, are normally located in public open space adjacent to roadways. General SUDS components can be incorporated into areas of private property and public open space land use. The dual use of land, where specific consideration is given to providing areas which are rarely inundated, is to be encouraged subject to appropriate consideration of safety issues. SR R. 4.0

76 Care should be taken when locating SUDS components in public open space areas that are also within the floodplain. Any SUDS located in a floodplain should not have an adverse effect on floodplain storage. Also, the risk to the operation and maintenance of the SUDS component, if flooding occurs, should be considered. SR R. 4.0

77 6. Construction Issues This chapter provides guidance on principal issues that relate to construction of SUDS systems. Guidance on the design and construction of SUDS (CIRIA, 2006) gives more detailed information on construction of SUDS systems. SUDS are a combination of civil engineering structures and landscaping practice. Due to the limited experience of building SUDS in the water industry, there are a number of key issues which need to be particularly considered as their construction requires a change in approach to some standard practices. Useful documents that provide particular guidance on SUDS construction issues are: Specification for highway works (Highways Agency et al, 1998) National building specification landscape specification ( Sewers for Scotland, 2 nd Edition (WRc, 2005) Water for Scotland (UKWIR, 2004) National SUDS Working Group framework document (NSWG, 2003) National SUDS Working Group interim code of practice (CIRIA, 2004) Further guidance on construction issues can be found in the following documents: CIRIA Report 609 (Wilson, Bray and Cooper, 2004) CIRIA Report 116 (Hewlett et al, 1997) CIRIA Book 10 (Hall, Hockin and Ellis, 1993) CIRIA Report C532 (Masters et al, 2001) Information is also given in the pollution prevention guidance produced by the Environment Agency and SEPA. Key aspects of SUDS construction require changes to conventional construction practices, and these are described in Box 6.1. Box 6.1 Construction considerations for SUDS 1. The phasing of construction may need to be modified to ensure constructed SUDS components operate properly. For many SUDS, the final construction should take place towards the end of the development programme, unless adequate provision can be made to protect the component. However it should be recognised that vegetation takes time to become established, and if they can be suitably protected, early construction will facilitate adoption by the organisation which is to own / maintain the drainage system. 2. The contractor and all relevant operatives should have an understanding of the mechanism and purpose of the SUDS components to ensure appropriate construction practice and protection is carried out. SR R. 4.0

78 Box 6.1 Construction considerations for SUDS (continued) 3. Traditional car parking and other paved areas are usually partially constructed during the initial stages of the development to provide storage and access on the site. However, if pervious surfaces are to be used in these areas, the construction process needs to be modified to prevent sediments from clogging the structure. Guidance on this is provided in Section If a pervious pavement is to be lined, the use of hardcore for structural purposes below the level of the liner can be applied. However the use of hardcore is not advised if infiltration is intended, due to the high proportion of fines. 5. Ground levels adjacent to pervious pavements must be set such so as to prevent any overland sediment washoff during high intensity rainfall events, or groundwater seepage during prolonged wet periods. 6. No traffic should be allowed on to the pavement if it is likely to introduce sediments onto the pavement surface from dusty or muddy areas. 7. Sensitive ground, such as chalk, may require the use of total exclusion zones to prevent compaction and other damage of the ground that will affect the infiltration performance of any infiltrating SUDS component. This may include protection from runoff during construction if the component is located at a low point on the site. 8. When excavating materials such as chalk, surfaces must not be smeared to avoid blocking fissures and pores. 9. The construction of swales, basins and ponds at an early stage in the construction will assist in managing runoff and help settle out the high volumes of sediments created during construction. However, complete reinstatement of these components will be required once construction is finished. It should be noted that the maintenance period is likely to stipulate establishment of landscaping vegetation, and sediment removal some time after site works have been completed. 10. The importance of good landscaping is emphasised. As SUDS systems are surface systems, attention to detail and aesthetics must be given a high priority. The seasonal and physical requirements of planting and establishing vegetation and prevention of soil erosion must be programmed appropriately. 11. Appropriate operative skills with an understanding of all aspects of vegetation should be obtained. In particular, the use of nutrients and mulches should be minimised and a particular soil mix will often be needed for the SUDS component. 12. Green roofs will not only require the consideration of appropriate season and skills, but the time needed to plant the roof areas needs to be specifically allowed for. 6.1 CONSTRUCTION ADVICE FOR PERVIOUS PAVEMENTS Fundamental to the construction of residential developments, is the use of the site road system to provide access within the site to assist in the construction process. Not only is this a construction necessity, it can also be a requirement of the highway authority, to mitigate the deposition of mud onto the public highway. It can also be a condition of planning. To prevent damage to the pervious pavement and its long term operation, protective measures need to be introduced. The following is suggested. SR R. 4.0

79 Following the construction of deeper services such as sewers (below formation), install the sub-base layers. In lieu of the top geotextile layer (if designed) and the bedding and paving, an additional tarmac running layer is installed. This provides support for construction and residential traffic and protects the permeable base of the road. Following the completion of construction activities on the dwellings, at the time when final wearing courses are normally applied, the tarmac layer is normally removed. Leaving the tarmac layer in-situ and puncturing it at a minimum of 1.0m centres is sometimes practised, but as this concentrates the flows, the risk of blockage is increased and is therefore not recommended. It also provides a horizontal plane reducing the friction resistance to horizontal forces (braking). Finally the geotextile, bedding layer and paving are installed as normal. Note that if the tarmac layer is retained, the difference between temporary and final surface level is significant and should be specifically considered in the design. If removed (sacrificial layer), the difference is much less. It also minimises hydrocarbon contamination of the runoff passing through pavement, if the final running surface is concrete blockwork. SR R. 4.0

80 SR R. 4.0

81 7. Drainage design and performance assessment Detail design and evaluation of the drainage system performance is presented in this chapter. The use of SUDS, together with discharge consent requirements, means that drainage design needs to consider a number of issues to ensure the system will operate without undue maintenance requirements and provide an environmentally appropriate solution. 7.1 INTRODUCTION In virtually all cases, development sites will not be allowed to install a traditional drainage system with unrestrained runoff to the receiving water or drainage system. The system will normally be required to provide: discharge rates and volumes no greater than those prior to development, attenuation storage, and a range of different treatment methods and volume reduction techniques. The principle being applied by current design criteria is to endeavour to produce runoff from a development that has similar discharge characteristics to the pre-development state. The introduction of these criteria and the use of a range of SUDS components should not be at the expense of increased maintenance requirements or an increase in the risk of failure. These criteria mean that a more complex approach is required for drainage design compared to traditional surface water pipe-based systems. However to try and provide assistance with the design, some rules of thumb and design charts are provided to assist with the design of the drainage system. It should be noted that, due to the variety of SUDS components and their different performance characteristics, it is likely that proof of the hydraulic adequacy of the system will usually require a hydraulic model to be used at the stage of detailed design. 7.2 DESIGN PROCESS Initial design The design process is normally undertaken in 2 stages. An initial analysis is carried out to assist in: development of a drainage strategy (required as part of a Drainage Assessment), assessing likely drainage land-take and an indication of the direct drainage costs, defining the flow conveyance through the site, and discussions with the local authority, maintenance stakeholders and the environmental regulator. Subsequent detailed design and evaluation will then be needed to demonstrate that the system performs as required. The initial assessment for a site should consider the stormwater constraints imposed by the catchment and the opportunities for use of appropriate SUDS (and in particular, infiltration techniques) based on the site characteristics. Distribution of storage across the site using a range of SUDS components is desirable, as this results in a reduction of the storage volume of the final attenuation structure and allows more flexibility in the design of a development. SR R. 4.0

82 An outline design of the conveyance process to be used across the site is necessary. This must include consideration of any downstream constraints created by receiving water levels. The plan area requirements of the storage components and their location should be considered, recognising that there might well be spatial constraints due to the need to meet housing density requirements. Figure 7.1 outlines the design process for the drainage system. The stages in yellow define the storage needs of the site. Initial evaluation of storage requirements can be found in Preliminary rainfall runoff management (Environment Agency, 2004). There is supporting software available for this guide available from the Environment Agency. Storage issues are discussed further in Section 7.5. Initial sizing of pipework can be based on a constant rainfall intensity of 30mm/hr to 50mm/h. The lower intensity would apply to the north of the country, while 50mm/hr would be more appropriate to the south and east with an allowance for climate change. Where pipework serves SUDS components which attenuate the runoff, these sizes will be conservative / over sized Detailed design Detailed drainage design and analysis involves the evaluation of the performance of the whole drainage system. Due to the range of SUDS components and their very different hydraulic characteristics, a detailed evaluation requires a detailed model that can replicate the system accurately. Approximations can be made depending on the analysis objective, but to obtain a complete picture of performance, all SUDS components and pipework need to be modelled. The model needs to be able to: predict flow rates and velocities for all parts of the system providing conveyance, predict depths of storage in all storage components, take account of infiltration rates and volumes, predict flood flows and routes due to exceedance of the system capacity, take account of downstream backwater effects. The detailed design process is shown on Figure 7.1 and comprises two possible sets of analyses. These are: use of a range of designs storms to: show that the system is robust (self cleansing and erosion velocities) prove an adequate level of service against flooding is provided, and demonstrate compliance to discharge consent stipulations. Use of an extreme and an annual rainfall time series to show that the drainage system is sustainable (as measured against greenfield runoff for the site) by assessing: annual infiltration volumes to groundwater extreme event flow rates and volumes discharged to the river frequent event flow rates and volumes discharged to the river The second set of assessments is (at present) not a requirement of drainage system performance evaluation. Figure 7.2 illustrates the various components of a drainage system and the various hydraulic performance issues. SR R. 4.0

83 Detailed system evaluation Initial system design Define catchment parameters: Area Topography Rainfall Soil Impervious area groundwater Define stormwater drainage concept for the site Establish receiving water levels and check for discharge constraints Outline network layout Pipes (size and location) SUDS components Outfall(s) Initial estimation of storage Performance compliance assessment Run Rainfall Design Events 1 yr 30 yr 100 yr Proof of compliance for: Level of service F 30yr (external) F 100yr (internal) Limiting discharge from site Q 1yr Q 100yr Limiting Volume from site Vol 100yr 6hr Operation and Maintenance V 1yr Treatment Treatment storage V t or 15mm of rainfall Model drainage system network and storage Greenfield runoff analysis Discharge rate Discharge volume Q 1yr Vol 100yr, 6hr Q 30yr Q 100yr Sustainability assessment Run Time Series rainfall Extreme series Annual series Performance comparison to Greenfield runoff Extreme series Volume of runoff Peak flow runoff Annual series Infiltration volume Volume of runoff Peak flow runoff Runoff control Attenuation storage Peak flow control, Q 1yr Q 30yr Q 100yr Long term storage Volume control Vol 100yr, 6hr Interception storage No discharge from sites 5-10mm of rainfall Q 1yr Peak discharge rate for 1 year return period F 30yr Flooding volume (and location) for 30 year return period Vol 100yr The volume of storage for 100 year return period V 1yr Velocity of conveyance systems for 1 year return period Figure 7.1 Schematic of the drainage design process SR R. 4.0

84 Figure 7.2 Drainage system performance requirements SR R. 4.0

85 The traditional approach to drainage system performance assessment since the introduction of the Wallingford Procedure in 1981 has been to use design storms of various return periods and durations. In general this provides a suitable method for assessing system performance and is much more computationally efficient than using time series rainfall. However it should be recognised that SUDS systems are more complex than traditional pipe systems and that design storms only provide an approximate assessment of their performance. In addition, due to the emphasis on sustainability and water quality, the use of time series rainfall provides much more information on the performance of the drainage system. In due course it is likely that larger developments are likely to need to demonstrate that their drainage proposals are sustainable. Each of the three types of system performance measurements is detailed in turn. Some guidance rules of thumb and figures are also to be provided to give some assistance with the initial design of the drainage system. The evaluation of sustainability will then be summarised briefly in outline for information purposes. 7.3 LEVEL OF SERVICE FLOOD PROTECTION The detailed design of the stormwater networks, including all SUDS components, should comply with the Sewers for Adoption 5 th Edition (WRc, 2001) criteria for: No flooding at 30 year event Consideration of extreme events (normally 100 year event) for flood routing and the location and extent of flooding. Sewers for Adoption 5 th Edition does not specify the return period of what constitutes an extreme event. It is normally a requirement to consider events of at least 100 years and take a risk based approach to check on the consequences of any failure that might take place. These criteria normally result in consideration of short high intensity storms at 30 year and 100 years to check on local flooding and overland flow. With traditional pipe based systems the critical duration is usually 15 to 30 minutes. However the use of SUDS tends to result in more storage provision and the critical duration event will generally be longer. As SUDS performance will vary between component types and is also influenced by topography, it will be important to run a range of durations to evaluate all parts of the drainage system. Maximum depths of flooding on storage areas will be dictated by longer critical duration events than for assessing the peak flow rate of flood flows. Consideration of flood depth is important to ensure that floor levels in properties are at a suitable level. The use of SUDS introduces a number of other design issues related to flooding that need to be considered. These are: Robustness of the component Implications of reduction in performance or failure SR R. 4.0

86 The use of ponds and basins may result in the construction of embankments for water retention, the possible failure of which needs to be considered. Other SUDS components may have a risk of reduced hydraulic performance over time. Specific consideration should be given to the probability and consequences of failure of any part of the drainage system. Flood routing due to possible failure should be evaluated for all system elements where this might have a significant impact. In particular, although the use of SUDS within the private curtilage is desirable to maximise the use of source control opportunities, there are particular risks related to possible change of use by the house-holder in the long term. A risk evaluation should be made to assist in the selection of appropriate design options Simple design methods Pipes As stated in Section the sizing of pipes is often based on a constant rainfall intensity ranging from 30 to 50mm/hr. It is normally found that this provides an adequate level of flood protection, when tested in a model. Gradients of pipes need to achieve 1m/s to meet the criterion set by Sewers for Adoption. A simple rule of thumb is that gradients can be the inverse of the diameter size, with a minimum gradient of 1:500. Thus a 150mm pipe can be laid to a minimum gradient of 1:150 and still achieve self cleansing velocity; however preferred (traditional) gradients of 1:DN/2.5 (i.e. 100mm pipe at 1:40) provide a more acceptable contingency against low flows, less than perfect workmanship and ground movements. These gradients may also meet the more relaxed velocity criteria (Chapter 3) of 0.3m/s downstream of SUDS systems which provide high levels of attenuation. However this will be a function of the design of the SUDS component and levels of SUDS workmanship that may be reasonably anticipated. This suggested criterion is only applicable where sediment loads are considered to be minimal to avoid risk of blockage. Pervious Pavements Standard pervious pavements constructed with granular media will provide around 100 year return period level of protection with a surcharge loading of an additional area equal to twice that of the paved area with quite constrained outflow rates. The level of service is lower in the wetter areas of the north and west. Swales Similar to pervious pavements, the storage volumes in swales (with relatively flat gradients) are large, so that even with highly constrained outflow discharge rates they provide very high levels of service against flooding. Linear ponds Linear ponds provide both the advantages of high volumes of temporary storage as well as excellent conveyance facility in flat terrains, thus generating a high level of service. 7.4 SYSTEM ROBUSTNESS OPERATION AND MAINTENANCE System performance for normal operation needs to be evaluated to ensure that maintenance requirements are minimal. Pipe flow requires to be self cleansing in accordance with Sewers for Adoption 5 th Edition, while flows in vegetated components (swales etc) should be as low as possible to prevent erosion as well as to maximise sediment deposition. SR R. 4.0

87 Pipe full flow should have a minimum velocity of 1m/s for a 1 year event where areas are served by a pipe and gully system. However, pipe systems serving pervious pavements and other components providing high levels of attenuation will rarely provide sufficient flow rate to achieve 1m/s for a 1 year event. Fortunately, the sediment yield from these SUDS components is very low and therefore reduced velocities are acceptable. It is suggested that these can be as low as 0.3 m/s. Flows in vegetated channels should preferably be in the region of m/s for frequent rainfall events, and be no more than 1m/s for a 1 year event. It should be recognised that drainage modelling of SUDS systems is unlikely to be as accurate as modelling pipe networks due to the effects of vegetation and soil types. Light liquid separators also act as effective sediment traps, as well as providing protection against hydrocarbon pollution. However where high sediment volumes are likely, it is normal to use proprietary products to deal with the sediment load prior to flows passing into the liquid separator. 7.5 STORAGE TO MEET DISCHARGE CONSENTS The design criteria for development runoff are normally based on runoff being controlled to mimic, as far as is practicable, the original greenfield runoff behaviour, unless the situation is such that some or all of these can be relaxed. This requires the greenfield runoff characteristics for the site to be estimated, so that the performance requirements of the drainage system can be set. There are two hydraulic characteristics of greenfield runoff that need to be considered. These are: greenfield runoff rate, and greenfield runoff volume. In addition, although not quantified, the water quality characteristics of the runoff should be addressed. These criteria result in the requirement for 4 types of storage. These are: Interception storage Attenuation storage Long term storage (for river flood protection) Treatment volume storage Interception storage Interception storage comprises the prevention of any runoff for the first 5 to 10mm of rainfall into the receiving water. In practice although certain parts of the site might achieve this criterion, it is unlikely that all paved runoff will be prevented from causing any discharge for these frequent events. However if greenfield runoff is to be replicated, then this criterion is an important one to achieve, as greenfield sites rarely produce any runoff for the many small rainfall events that occur. Any interception storage provided reduces the attenuation storage requirement and also, depending on how the long term storage is provided, reduces the long term storage volume requirement. SR R. 4.0

88 Attenuation storage Having determined the peak rates of runoff for greenfield conditions, the attenuation storage volumes needed to meet this criterion can be determined. This can be done by using: a drainage system computer model, or estimate an initial assessment using the Environment Agency guide (Environment Agency, 2004) and supporting software tool. The guide is in the form of look-up tables and graphs to provide an initial estimate of storage volume, or a coarse rule of thumb (Table 7.4) which should be treated with extreme caution, but which provides an indication of the order of magnitude of attenuation storage needed as a function of soil type. Appendix 3 provides additional information on alternative storage volumes. Long term storage Long term storage is the term given to the additional stormwater runoff volume which is generated by the development compared to what used to be discharged from the greenfield site. The water stored in this location should preferably be infiltrated or discharged to the receiving river at a rate of less than 2l/s/ha. The reason for this requirement is to limit any significant increase in flood risk in the river downstream during extreme events. Therefore activation of the storage need not take place except during extreme rainfall although this is may be difficult to achieve in practice. This means that the flood storage area need not necessarily be in the form of a drainage structure. Good planning design of the site will enable an appropriate area to be flooded when necessary. Long term storage is effectively a component of the attenuation storage volume. Appendix 2 provides an illustration of the analysis carried out to calculate the various storage volumes required. Treatment storage Treatment storage has been described earlier in Section 3.4 and is normally provided as the permanent pool volume of the attenuation pond(s). Figure 7.3 shows the various components of storage and illustrates how the various storage components integrate Simple design methods interception storage The criterion for interception storage is to provide interception of all runoff for the first 5 to 10mm. As the first millimetre of rainfall can be assumed as just wetting the ground, Table 7.1 summarises the minimum and desirable volumes of runoff which should be intercepted. This is based on an assumption of a paved proportion of 70% and a runoff factor of 60% (which is appropriate for small events). Table 7.1 Interception volume for a typical development Interception volume from 5mm of rainfall (m 3 /ha) Interception volume from 10mm of rainfall (m 3 /ha) SR R. 4.0

89 Swales It should be noted that if mini swales are provided along 50% of the road length on each side of the road, that 100mm depth of storage provides more than 15mm of effective storage for road runoff. This provides very significant water quality benefits without a large take-up of land. Figure 7.3 Storage components for stormwater runoff control Simple design methods long term storage Long term storage is a simple calculation of storage volume based on the 100 year 6 hour event. Figure 7.4 shows the 100 year 6 hour map of UK and Figures 7.5 and 7.6 show the additional runoff volume per hectare for a range of soil types and level of impermeability (extent of pavement and roofs) of the site. Assuming that Figure 7.5 applies, (that no runoff takes place from pervious areas after development), the volumes in Table 7.2 can be derived. This provides an estimate of the long term storage based on 80% runoff for a 70% level of impermeability. SR R. 4.0

90 Table 7.2 Long term storage for a typical development SOIL type Storage volume (m 3 /ha) Greenfield runoff is generally much less than the runoff generated after the site has been developed. This difference becomes proportionally less as rainfall events become more extreme such that heavy clay catchments generate similar greenfield runoff volumes to that produced by the development. However greenfield areas with permeable soils always generate much less runoff events for extreme events. The estimation of runoff volume from greenfield areas is based on FSSR 16 (NERC, 1985), which is detailed in Appendix 1. However this closely approximates to an assumption that runoff volume is equal to the standard percentage runoff (SPR) value for the given soil type. Table 7.3 gives the SPR value for each of the 5 soil types used in the formula. See Table 7.6 for a description of the different SOIL types. Table 7.3 SPR values for different SOIL values (from FSR) SPR value SOIL (runoff factor) As runoff is approximately proportional to rainfall depth, it is not easy to design a drainage system to make it generate the same greenfield runoff volume for every rainfall event. Therefore for simplicity this criterion is applied to the 100 year 6 hour rainfall. There is usually no criterion for frequent rainfall events as there is effectively no direct runoff from these events on greenfield catchments. The use of 6 hours, rather than another time period, is selected based on the fact that small catchments have critical rainfall durations of around 4 to 6 hours. A development in a small catchment is likely to have significantly more impact than one in a large catchment and therefore provides some justification for use of this event duration. Figure 7.4 provides the values of 100 year 60 hour rainfall depths for the 7 hydrological FSR regions across the UK. Calculation of long term storage is simple, but requires a decision as to whether the unpaved areas are assumed to contribute runoff or not. The following formula allows assumptions to be made as to whether some or all of the paved and pervious areas contribute runoff. The formula assumes that only 80% runoff occurs from paved areas, but 100% runoff can be assumed if it is felt that a more conservative assumption is needed. PIMP PIMP Vol xs = RD. A.10 ( α 0.8) + 1 ( β. SPR ) SPR (7.1) SR R. 4.0

91 Where: Vol XS is the extra runoff volume (m 3 ) of development runoff over greenfield runoff RD is the rainfall depth for the 100 year, 6 hour event (mm) PIMP is the impermeable area as a percentage of the total area (values from 0 to 100) A is the area of the site (ha) SPR is the SPR value for the relevant SOIL type α is the proportion of paved area draining to the network or directly to the river (values from 0 to 1) β is the proportion of pervious area draining to the network or directly to the river (values from 0 to 1) This formula can be simplified by assuming that either all the pervious runoff continues to contribute as it did in the greenfield conditions, or that none of it contributes. The difference in additional runoff volume calculated is quite significant even for quite heavily developed catchments. If all the paved area is assumed to drain to the network and all the pervious areas are landscaped not to enter the drainage system or river, this formula simplifies to: PIMP Vol xs = RD. A SPR (Eqn. 7.2) 100 But where all pervious areas are assumed to continue to drain to the river or network the formula becomes: PIMP PIMP Vol xs = RD. A SPR (Eqn. 7.3) Figures 7.5 and 7.6 illustrate the difference in runoff volume for these two extremes (fully disconnected / fully connected pervious surfaces) for the five different soil types for any development density. To obtain a volume the y-axis value is multiplied by the catchment area and the rainfall depth. These graphs demonstrate the great effect soil type has on long term storage volume, the importance of using infiltration to disconnect impermeable areas from the drainage network and the need to be efficient in designing the general landscape to prevent runoff from pervious areas Simple design methods Attenuation storage Attenuation storage is a function of climate, soil and site development characteristics. It is therefore impossible to provide a simple guidance look up table which is accurate. However for the purpose of providing an indication of the order of magnitude, Table 7.4 provides an indication of the typical volumes of storage that are likely to be required as attenuation storage. It is stressed that these estimates are only to be treated as being indicative and more detailed analysis will be needed when the drainage system is designed. Appendix 3 provides more information for attenuation storage volumes. Table 7.4 is followed by an explanation of the process for calculating a more accurate estimate. SR R. 4.0

92 82 mm mm 70 mm 63 mm 9 61 mm 60 mm 55 mm 8 51 mm kilometers Irish grid 1 M hr - Rainfall depth National grid Figure year 6 hour rainfall depths for hydrological FSR regions across the UK SR R. 4.0

93 8 7 6 Vol.(m 3 /ha.mm) Proportion of impermeable surface SOIL 1 SOIL 2 SOIL 3 SOIL 4 SOIL 5 Figure 7.5 Additional runoff volume caused by development where all pervious areas are assumed not to drain to the drainage network (Eqn. 7.2) Vol.(m 3 /ha.mm) Proportion of impermeable surface SOIL 1 SOIL 2 SOIL 3 SOIL 4 SOIL 5 Figure 7.6 Additional runoff caused by developments where all pervious areas are assumed to drain to the drainage network (Eqn. 7.3) SR R. 4.0

94 Table 7.4 Approximate attenuation storage volumes required for a typical site for a 100 year event SOIL type Volume of storage (m 3 /ha) These estimates are based on average runoff control rates and present day rainfall. In the case of SOIL type 1, although the greenfield runoff rate is usually calculated to be less than 1l/s/ha, this figure is usually used as a minimum for reasons of practicality. It can be seen from these figures that if it is assumed that the attenuation volume is on average 1m deep, the catchment proportion needed for storage ranges from 4.5% for SOIL type 1 down to 2.5% for SOIL type 5. This draws attention to the needed to distribute storage as much as possible in upstream components such as pervious pavements. Subject to the design approach, the distribution of storage can proportionally reduce the volume required by the final attenuation structure. In addition the provision of interception and long term storage will also reduce the attenuation requirements and therefore the land take of such structures. The calculation of attenuation volume requires an analysis of greenfield runoff rate. Greenfield runoff rate The regulatory authority will normally require the rate of runoff from a development for a range of return periods up to and including the 1 in 100 year return period to be no greater than the greenfield runoff rate for the same range of return periods. The recommended methods for estimating greenfield runoff rates for development sites are outlined in Table 7.5. Appendix 1 provides an example of determining the runoff characteristics of a greenfield site. Table 7.5 Recommended greenfield runoff rate estimation (SUDS Interim Code of Practice) Development Method size 0 50ha The Institute of Hydrology Report 124 Flood estimation for small catchments (1994) (Institute of Hydrology, 1994) should be used to determine peak greenfield runoff rates. Where developments are smaller than 50ha, the analysis for determining greenfield discharge rate should use 50ha in the formula but linearly interpolate the flow rate value based on the ratio of the development area to 50ha. FSSR 2 and FSSR 14 regional growth curve factors should be used to calculate greenfield peak flow rates for 1 and 100 year return periods ha IH Report 124 should be used to calculate greenfield peak flow rates. Regional growth factors to be applied. SR R. 4.0

95 Table 7.5 Recommended greenfield runoff rate estimation (SUDS Interim Code of Practice) continued Development size Above 200ha Method IH Report 124 can be used for catchments that are much larger than 200ha. However, for schemes of this size it is recommended that the Flood Estimation Handbook (FEH) (Institute of Hydrology, 1999) should be applied. Both the statistical approach and the unit hydrograph approach should be used to calculate peak flow rates. However, where FEH is not considered appropriate for the calculation of greenfield runoff for the development site, for whatever reasons, IH 124 should be used. Other methods for estimating greenfield runoff are available, for example MAFF Report 345 (MAFF, 1981). A brief overview and comparison of these other methods is given in Kellagher (2004). It should be recognised that the level of accuracy for predicting the runoff rate for any undeveloped small catchment, using IH 124 or any other formula, is limited. The use of this formula is only important in that it provides a consistent, easy to use method for assessing appropriate limiting discharge control volumes. The equation given in IH 124 is based on the Flood Studies Report (Institute of Hydrology, 1975) work with measurements from a few additional catchments. It predicts a value for the mean annual flood, Q BAR. Q BAR = AREA 0.89 SAAR 1.17 SPR 2.17 (7.4) where: Q BAR is the mean annual flood flow from a rural catchment in m 3 /s AREA is the area of the catchment in km 2. SAAR is the standard average annual rainfall for the period 1941 to 1970 in mm. SPR* is the SPR value for the SOIL index, which is a composite index determined from soil survey maps that accompany the Flood Studies Report. Details are given in Table 7.6. * The term SOIL is normally used in this equation, to refer to the equivalent SPR value. The term SPR has therefore been substituted to reduce the potential for confusion. Table 7.6 SOIL Index descriptions (from the Flood Studies Report, Institute of Hydrology, 1975) SOIL Index Description of soil type 1 Well-drained permeable sandy or loamy soils and shallow analogues over highly permeable limestone, chalk, limestone or related drifts. Earthy peat soils drained by dikes or pumps. Less permeable loamy over clayey soils on plateaux adjacent to very permeable soils in valleys. 2 Very permeable soils with shallow ground water. Permeable soils over rock or fragipan, commonly on slopes in Western Britain associated with smaller areas of less permeable wet soils. Moderately permeable soils, some with slowly permeable subsoils. SR R. 4.0

96 Table 7.6 SOIL Index descriptions (from the Flood Studies Report, Institute of Hydrology, 1975) continued SOIL Index Description of soil type 3 Relatively impermeable soils in boulder and sedimentary clays, and in alluvium, especially in Eastern England. Permeable soils with shallow groundwater in low lying areas. Mixed areas of permeable and impermeable soils in approximately equal proportions. 4 Clayey or loamy over clayey soils with an impermeable layer at shallow depth. 5 Soils of the wet uplands: (i) with peaty or humose surface horizons and impermeable layers at shallow depth, (ii) deep raw peat associated with gentle upland slopes or basin sites, (iii) bare rock cliffs and screes, and (iv) shallow impermeable rocky soils on steep slopes. What should be particularly noted is that Q BAR is virtually proportional to area and annual rainfall depth, but is significantly influenced by soil type. Q BAR is factored using the regional growth curves for UK to produce peak flow rates for other return periods. This information is available in Flood Studies Supplementary Reports 2 and 14 (NERC, 1977 and 1987) produced by the Institute of Hydrology. Table 7.7 summarises the factors for the 1 year and 100 year return periods for each hydrological region. Table 7.7 Hydrological region factors for Q BAR Region Factor for 1 year return period Factor for 100 year return period It is advised that the formula for determining the peak greenfield runoff rate should not be applied to areas that are less than 50 hectares. As many developments are smaller than this size, this constraint is avoided by calculating Q BAR for 50 hectares and linearly extrapolating flow rates for smaller areas. This is a reasonably conservative application of the formula. Table 7.8 Typical values of Q BAR for SAAR of 700mm and a site of 50ha SOIL type SOIL type 1 SOIL type 2 SOIL type 3 SOIL type 4 SOIL type 5 Q BAR (l/s) 8.4* Q BAR (l/s/ha) 0.2* A minimum value for Q BAR of 1l/s/ha is normally applied for practicality. SR R. 4.0

97 A limitation of the IH 124 method is the lack of a slope function. This may be particularly significant for small steep catchments and therefore in these instances the use of appropriate alternative methods for estimating greenfield runoff rates, such as MAFF 345, might be considered. If IH 124 is applied the results may be overly conservative Simple design methods - Runoff treatment The final form of stormwater storage is the provision of treatment storage which has been described in Section 3.4 and is revisited here. Theoretically this can be provided by a number of SUDS components, but in practice it usually refers to the permanent pool volume in an attenuation storage pond. The objective of the permanent pool is to enable sedimentation to take place and also make use of any biological or chemical processes that might usefully treat the runoff. The size of the storage is often calculated using Equation 7.5 and then factored by as much as four times, though this is not now regarded as appropriate in most cases. It is currently recommended that the permanent pool should normally be approximately a factor of one times the treatment volume. V t (m 3 /ha) = 9 x D(SPR/2 + (1 - SPR/2) x I) (7.5) Where: I = fraction of the area which is impervious D = M5-60 rainfall depth (5 Year Return, 60 minute duration) SPR = SPR value for SOIL classification (from Flood Studies Report or the Wallingford Procedure WRAP map) V t is thus a function of local hydrological characteristics, soil type and the level of impermeability of the catchment. Depending on where this formula is applied, this volume approximates to the runoff from rainfall of between 12 to 20mm of rainfall. The intention is that this provides at least 24 hour retention for 90% of all rainfall events. The treatment volume for 15mm of rainfall at 80% runoff and an impermeable fraction of 70%, with all the paved area needing treatment is 84m 3 /ha. Current research suggests that a small base flow might be useful in preventing anaerobic conditions developing, particularly in hot dry periods, and that the retention volume can be too large as much as too small. Although the treatment volume will have been determined at initial design stage, a check needs to be made that the relevant assumptions have not changed in the evolution of the design. Detailed design needs to consider the hydraulics of the final arrangement to ensure that no jetting or short circuiting of flows takes place. 7.6 REFINEMENT OF STORAGE VOLUME AND SYSTEM PERFORMANCE ASSESSMENT Although initial analyses methods provided a quick assessment of the various storage requirements needed for managing stormwater runoff, a detailed model will be needed to take into account the head-discharge relationship of all the storage components. Where there is a risk of backwater effects, these need to be considered, particularly with outfalls to rivers. It should be noted that the critical duration of storage systems is often SR R. 4.0

98 of the order of 12 to 24 hours. Rivers rise to their maximum flood level in similar events and therefore there is likely to be a significant degree of dependency. Before carrying out a joint probability assessment, it is suggested that an initial simple approach is taken by assuming that the water level in the receiving stream is at its maximum level for the critical duration event for the pond. A more detailed joint probability assessment may be needed if the implications for the storage system volume are very significant. Compliance to the criteria of 1 and 100 year discharge limits needs to be demonstrated by running the model with a range of rainfall events of various durations for each return period and showing that discharge rates do not exceed the consent values Long term storage design Where long term storage is provided as a temporary floodable area, consideration needs to be given to ownership and clean up responsibilities as well as the hydraulic drainage mechanisms. Under-drainage should be provided to ensure a relatively rapid return to its original state is achieved if public open space is flooded. Compliance should be demonstrated using a detailed model by showing that the long term storage volume is retained for the 100 year event. The critical duration event is unlikely to be 6 hours, but likely to be similar to the critical duration for determining the attenuation storage volume. 7.7 FUTURE ADDITIONAL MEASURES OF SUSTAINABILITY OF DRAINAGE SYSTEMS Although not required at present, the growing emphasis on water conservation, water quality and infiltration for groundwater replenishment, means that there will be a need to use time series rainfall to show the system characteristics on these issues. The use of time series rainfall also has the benefit of showing the actual performance of the system as the different hydraulic characteristics of the SUDS components will be more accurately represented. This type of assessment will allow a more appropriate comparison to be made between greenfield conditions and the development drainage system for all storms, thus allowing a more accurate evaluation as to whether greenfield conditions have been reasonably replicated. It is therefore anticipated that time series rainfall will become a feature of detailed design in due course, particularly for large developments. SR R. 4.0

99 8. Safety, Amenity and the Environment This chapter looks at the opportunities SUDS provide for enhancing the amenity in a development and other environmental benefits. As many SUDS components involve storage of water which is accessible by the public, safety is also an important issue to address. 8.1 INTRODUCTION The design of SUDS components must take into account a range of factors which may require trade-off compromises to be made to ensure that all the constraints of safety, amenity and the environment are all satisfied. Issues affecting these choices include maintenance costs, performance failure risk and liability. The chapter is structured to look at each of the topic areas separately and then a summary is provided with a matrix to provide guidance on issues where decisions need to be made where there is potential conflict. 8.2 AMENITY Amenity is a general term that it is useful to define. The Oxford English Dictionary (Oxford University Press, 1989) defines amenity as: Something that contributes to physical or material comfort, A feature that increases attractiveness or value especially of a piece of real estate or a geographical location. As environment has been separated out, amenity can be considered to comprise aesthetics and potential for public use Aesthetics All SUDS components, except underground infiltration systems, have elements of amenity, in that they are all visible to the public and therefore careful consideration should be given to their aesthetic impact. For example pervious pavements can make use of coloured blockwork to provide patterns as well as drawing attention to the purpose of the area. Thus car parks can differentiate parking bays and traffic lanes, or an area can be laid to provide artistic relief. There are very few negative implications for making use of this facility, although the concept of what is artistic is subjective and therefore may not be universally approved. Vegetative components such as swales and ponds have considerable potential for being aesthetically enhanced. However the decisions relating to vegetation use will have implications for maintenance and possibly performance of the system. The ecological value of any component is dependent on the choices made. It should be noted that what might be considered to be aesthetically very pleasing to the public is often not an optimum solution for ecological diversity. Another aspect on the choice of vegetation is the need for a safe environment. The design of ponds may involve the use of barrier planting which might be located within the pond margin or immediately adjacent to the pond edge. It is also considered SR R. 4.0

100 important to have an open and clear view of a pond, not just because open water looks attractive, but also because it is considered to be a good safety feature, as children can be seen and appropriate action can be taken Active and passive public areas Public open spaces are provided for people to enjoy in a variety of ways. These range from dog walking through to football and play areas. Open space can therefore be considered as being either active or passive areas. As ponds and basins can take up a significant part of the public open space, consideration needs to be made with regards to dual use of the land and activities that will be carried out adjacent to the SUDS component. It is often suggested that large basin areas, that are normally dry, should be available for public use to maximise the value of the land. In practice the location of SUDS on an active area such as a football pitch has implications for which maintenance responsibility would have to be defined and agreed. The drainage authority who has responsibility for the basin is unlikely to want to be involved with areas which require high levels of maintenance, such as weekly grass cutting, for recreational reasons. In addition, the activity itself may detract from the effectiveness of the SUDS component and there may be safety aspects with regard to the pollutants in sediments. Active play areas located immediately adjacent to ponds might be considered to be unsafe, especially where it is targeted at young children. There are also some concerns with regards to passive use of these areas. Due to the pressure of green space availability, dog walking tends to result in faeces adjacent to the pond with implications for both health and pollution. Even the provision of park benches encourages litter and food which can result in additional nutrients, encouragement of the local rat population and increased maintenance in ensuring a clean and tidy area. It can be seen therefore that the design of the area adjacent to ponds and basins needs to be carefully carried out to encourage activities that are safe and discourage activities which are either unsafe or might cause operational problems. 8.3 ENVIRONMENT The meaning of environment can be very wide. In this context environment refers primarily to the ecological and water quality benefits of SUDS, both within the SUDS component, adjacent to it and the receiving water body. Good ecology is measured in terms of bio-diversity of both flora (vegetation) and fauna (normally macroinvertebrates, but also insects and birds). Water quality performance of a SUDS component is not just a function of the design of the component, but is related to the hydrological conditions, both dry periods and rainfall events, as well as the land that it drains. There are ways in which the water quality can be optimised and these will have implications for the ecology. The water quality of stormwater using normal SUDS design techniques for a site will never be pure. Theoretically, given enough attention and space, the water quality at the outfall can be very good. However this is unlikely to be a requirement for most SUDS drainage systems for new developments, though designs will aim to meet the requirements of the Water Framework Directive (2000/60/EC). It should be recognised SR R. 4.0

101 that the water quality in the SUDS components will affect the diversity of the flora and fauna. Although there has been a limited amount research on the subject, it is encouraging to note that the diversity can be quite high with the standard application of SUDS systems Optimising ecological diversity The ecological value of most SUDS components is relatively small. Vegetative systems include swales, ponds and basins, and of these, ponds are by far the most important with regards to ecological value for an area. In fact ponds, natural or otherwise, provide a much higher level of diversity compared to other aquatic environments (HR Wallingford, 2003). Swales can become bio-retention areas where the topography is relatively flat. However marshy areas, with unrestrained vegetation growth, look untidy and are unlikely to be aesthetically acceptable. The traditional approach of having regular mowing of a grassed component results in poor bio-diversity but generally meets with approval by the public. Basins, when they are not directly within public view, such as in the centre of roundabouts, can be attended to far less frequently. This has advantages for maintenance costs as well as the ecology. However basins located in full public view will probably need to have a more managed appearance. Much of the diversity in the aquatic environment is around the margin of the pond, particularly in the shallow water zone. Thus to maximise the value of a pond in terms of the ecology requires a virtually horizontal border at the pond edge and a shallow zone immediately within the pond. This border should not be engineered to be uniformly smooth. Local humps and bumps and a variety of materials such as wood and rocks all adds to the potential for diversity. Another advantage of this approach is that a muddy periphery will deter most people from walking into the pond. The disadvantage of this design is the inefficient use of land in meeting the hydraulic requirements for attenuation of the stormwater runoff. Water quality treatment processes are maximised in the shallow zone, although as sedimentation is the most important process, the shallow margin only provides secondary water quality benefits. The vegetation around the pond needs to be considered in terms of bio-diversity value and also as providing a barrier to ingress into the pond. These plants are not only diverse in themselves, but also provide the habitat for the macro-invertebrates and other animals. A critical problem for urban stormwater ponds is the prevalence of invasive species. Once these take hold they can dominate the environment and wipe out diversity in the area. These species can be introduced intentionally or otherwise, and the proportion of urban ponds with this problem is of considerable concern. Box 8.1 advises on invasive species to be avoided. The recommended approach is for allowing natural colonisation of the pond from native species. Where it is located next to natural open space, this colonisation can be quite rapid. Unfortunately this is rarely an acceptable solution as the adoption of the pond SR R. 4.0

102 will often require establishment of the vegetation. Also, the need to provide a barrier means that certain species will need to be encouraged to grow. Appropriate skills need to be used to ensure that pond design and planting is optimised. Expert guidance is available from a number of sources and information on plants that are local to any area is available from HR Wallingford (2003). Box 8.1 Invasive alien wetland plants which pose a high risk to the environment. These plants should be excluded from all SUDS planting schemes (HR Wallingford, 2003) New Zealand Pigmy weed (Crassula helmsii) Parrot s-feather (Myriophyllum aquaticum) Floating Pennywort (Hydrocotlye ranunculoides) Water fern (e.g. Azolla filiculoides and close relatives) Box 8.2 Issues associated with planting SUDS ponds (HR Wallingford, 2003) Ensure that the contract for all planting up of schemes specifies the requirement for native species of local provenance. Where possible work with local plant suppliers to develop appropriate ranges of native plant species of local provenance. Include only common species (unless the scheme is part of a recognised conservation project to protect populations of a particular uncommon plant). Include only species which are characteristic of local ponds. Focus particularly on the more inconspicuous, but ecologically valuable, aquatic grasses, especially Creeping Bent and the Sweet-grasses (Glyceria species) which provide good invertebrate habitats. Ensure that an experienced botanist assesses planting schemes before projects are signedoff to check what has actually been planted (as opposed to specified). Check again for the presence of invasive species after one year. Contractors should be responsible for removing any unspecified material and make good any damage incurred to other plants. Check aquatic suppliers premises in order to ensure that highly invasive species are not rampant and growing wild in their propagating areas (as has been observed at some sites). Multiple ponds provide an opportunity to alter both the depth and shape of the pond, but more importantly, the water quality process of ponds in series results in a much better effluent quality and creates different communities of species. An additional advantage is that the maintenance activities such as vegetation control and sediment extraction, which cause some degree of disruption, will have much reduced impact on the pond community. A second advantage is that shock loads of pollution, such as accidental spillage, can be accommodated more effectively Water quality The processes by which water quality improvements are obtained from SUDS systems range from biodegradation, sedimentation and filtration through to phytolosis. The most important of these are sedimentation and filtration. Different levels of water quality improvement are provided by different SUDS components. SR R. 4.0

103 Appropriate use of SUDS can result in discharges with sediment and BOD concentrations of the order of 20mg/l. Further polishing of the effluent to a higher standard is achievable, but at the cost of additional land take and SUDS components. 8.4 SAFETY Safety is a crucial element for all engineering design including SUDS. This section looks at safety in terms of each SUDS element. In some instances guidance from appropriate authorities is not clear or contradictory, as there can be a difference of opinion as to the best way to maximise safety. However the guidance is usually straightforward and consistent. It is presumed that all issues pertaining to CDM regulations are followed Swales and Bio-retention Swales are shallow ditches that are normally placed besides roads, and the preferred mechanism for accepting runoff is sheet flow. In not providing kerbs there is some risk of damage to the swale edge either from deliberate parking or accidentally leaving the carriageway. In the case of the latter, the implication of vehicle speeds should be considered. For these reasons, the use of kerbs is probably advisable in many instances, but this means that there is a risk of erosion in passing flows from the road into the swale. Trials, which have used multiple entries of small pipes through the kerb using shallow water collection components, have generally been shown not to work. The most effective way of introducing runoff into the swale is to use drop kerbs at frequent intervals. The spacing of these gaps should be frequent such that only half the length of the road edge is kerbed. The chance of drowning in swales is remote if they are primarily designed as conveyance components. Components that are designed to store stormwater for a significant period of time, such as under-drained swales, may need to be evaluated in terms of the risk of drowning even though depths of water are usually very shallow Pervious pavements Pervious pavements are no different to ordinary road surfaces in terms of safety except they have the advantage of not having standing water on a flat pavement. Pervious pavements are often designed with a geo-textile layer close below the block work. Vehicles, particularly heavy goods vehicles, which brake heavily or turn sharply, can disrupt the pavement as the geo-textile provides a plane of reduced friction allowing blocks to move. Appropriate design can minimise this issue Ponds Health The risks associated with ponds are readily acknowledged as being both health related in terms of disease vectors and that of drowning. Aspects that relate to health include: West Nile virus Malaria SR R. 4.0

104 Weils disease blue-green algae infected cuts from glass or other sharp objects though some of these are not endemic in UK. One of the difficulties facing designers is that there is no complete checklist to consider all the possible health-related issues. A good example is that of West Nile virus which has recently become a problem in America. Whereas previously good practice involved the use of plenty of vegetation, this new health problem is exacerbated by the existence of vegetation, and ponds are being modified to address this problem. In addition to these relatively serious aspects, there are nuisance elements such as midges and flies to consider. In general terms where water is free moving, stirred by winds or induced currents, and low in nutrients, vector water-borne diseases are minimised. Appropriate fencing This is a subject area which is heavily debated with positions held by experts that are sometimes contradictory. At one extreme there is the provision of fencing such that access would be extremely difficult to achieve even by fit athletic persons. At the other extreme is the provision of no fencing at all. Between these two points of view there are two further recommendations which are; to use a one metre high fence, or just an indicative barrier which can be stepped over. Overall, the choice of whether to fence a pond should be based on a risk assessment for the location and communication with appropriate stakeholders. The arguments related to the choice of fence are as follows. 1. A very high fence is likely to be overcome in some way because it is there to be beaten. In today's society there seems to be a tendency to treat all symbols of authority as a challenge. 2. No fence, on the other hand, prevents no challenge and soft measures of persuasion such as barrier planting and educational signs can provide good deterrence. However, a two-year-old toddler would tend not to be dissuaded by such features, but parents would recognise the risk and should act accordingly. 3. A one metre high fence addresses the issue of preventing small children from entering the pond, but by the age four or five it is unlikely that such a fence would prevent their access. The one metre fence would not be seen as a strong sign of authority, but allows adults to gain easy access where there is a need to provide assistance or retrieve some personal object. 4. A mini fence provides no real obstruction for small children but does give a message to adults and older children which is not a challenge. It is also much less obtrusive and therefore does not detract from the amenity value of the pond. Examples of most of these options have been built. There has only been one recorded fatality in UK in a SUDS pond, so there is no statistical basis upon which to make a judgement. Inspection of the use of ponds as an amenity shows that mothers and toddlers will regularly use paths around a pond to go for walks. Although they recognise that the pond is a potential risk, few demand the provision of fences. Conversely where fences exist, particularly those that do not look attractive, this often results in considerable local disapproval due to the perceived risks associated with the SR R. 4.0

105 pond. It would seem therefore that the no fence solution might be the best option with as much emphasis as possible on education and other forms of safety provision. However each site will need to have the risks evaluated and would need to take into account the culture of the local population and its likely age profile, as well as the physical factors of location and pond design. Pond configuration It is generally acknowledged that a safer pond is one that is more distant from housing, but is overlooked. Unfortunately in high density developments, the maximum distance possible may only be tens of metres. Additional deterrence can also be provided by locating the pond on the far side of a road. Pond location is unlikely to be a matter of great choice. Pond ground slopes The slope of the ground adjacent to the water's edge and immediately within the pond needs to be kept as flat as possible to prevent people from slipping into it. The acknowledged compromise between the trade-off between land take and safety is to use maximum gradients in the region of 1:3 or 1:4. Steeper slopes might be acceptable at higher levels when the frequency of the water at that level is rare. Water levels above the 2 or 10 year event may be regarded as being sufficiently infrequent to provide a minimal risk of drowning for the brief time at which water is at this level or above. Pond depths It is generally felt that deep water provides a much higher risk of drowning than shallow water. However as the minimum water depth required to prevent aquatic vegetation growth is over one metre, the difference in level of risk in designing an attenuation pond to have a maximum water level of two or three metres does not significantly change the risk of drowning. The important aspect is to provide prevention measures which deter entry into deep water. Lifesaving equipment The provision of lifebelts is normally recommended along with suitable signage. These need to be strategically located and sufficiently frequent to be useful. However perhaps the most useful aspect of this strategy is their existence, which draws attention to the potential risk of drowning and is a simple way to get this message across to the public. Vandalism may be a problem with providing this type of safety equipment. Educational information In addition to providing warnings of risk aspects related to the pond, information boards provide an opportunity for educating the public with regards to the benefits that the pond provides. Whether all the potential hazards relating to health are listed as well or a general warning given against swimming and fishing, depends to some extent on the likely health risks and the community where the pond is located. Pond management Where there is the requirement for no swimming and fishing to take place, this needs to be backed up by a management plan. Once these practices develop, it is more difficult to prevent recurrence. Therefore sufficiently regular visits should be made to ensure that unsafe practices are not taking place. SR R. 4.0

106 Instances of drowning often relate to weather conditions, particularly hot weather. Where there is a possibility that a swim to cool down may seem attractive, particular vigilance is required, especially if consumption of alcohol might be associated with the location Detention basins There is a difference of opinion as to whether detention basins are more or less of the risk than ponds. Clearly detention basins only retain water for a brief period of time over the year, and that the depth of retention for the majority of events will be shallow. Therefore if risk of drowning relates to the period when water is available to drown in and its depth, then detention basins can be regarded as a much lower risk than ponds. As detention basins take significantly less land than ponds their use would therefore seem to be attractive. Apart from being intrinsically less attractive than ponds, the use of barrier planting to dissuade access into it is less easily addressed. The location in the basin as to where they should be grown and the additional maintenance implications created, makes barrier planting less desirable. It is therefore perceived by some authorities that detention basins provide no less a safety risk than ponds. This means that many of the issues regarding safety provision for ponds apply to the use of detention basins Filter trenches Filter trenches with granular material on the side of the road is commonly used on major roads, but not in residential areas. If they were to be used the only particular risk related issue is the effects of stone scatter on the road. These risks are not new as these drainage systems have been used for many years. 8.5 LIABILITY Liability related to drowning is a high priority for any owner of a site with hazards. Provision needs to be made such that in the unfortunate occurrence of a drowning taking place, that it can be shown that every reasonable effort had been made to prevent such an event happening. These provisions can include: Suitable (pond) design Consideration of distance and adjacent land use Educational warning signs Appropriate management Lifesaving equipment Appropriate fencing In addition to these provisions, the responsibility in law for liability is affected by whether the property is private, or whether the public have a right of access. These various issues are briefly discussed below. Legal disputes relating to personal injury and negligence are an important consideration for both the design of the pond and other safety provisions. The legal requirements relating to safety that are applicable to ponds are the Occupiers Liability Acts of 1957 and 1984 and the Countryside and Rights of Way Act SR R. 4.0

107 The law recognises that there is a difference in degree of liability between areas with a public right of access and private property. It also recognises that certain activities, such as fishing, implicitly accept the risks associated with open water. It is therefore likely that all such SUDS components will have to be located in public open space. 8.6 SUMMARY Table 8.1 summaries the issues raised in this chapter. Items highlighted in red indicate a potentially contradictory requirement while those in yellow are less important issues for compromise. Green indicates areas where design criteria should not result in contradictory requirements. Table 8.1 Summary of safety, amenity and environmental factors and issues requiring compromise in design Pond Design Aesthetics Safety Environment Hydraulic Water Quality Close- Backwater enhances Near natural Location Distant effects if close - property rural land to river values Depth Vegetation Greater than 1m to maintain open water Variety limited to extent Minimise Barrier planting Extensive shallow zone with variations Natural local plants - Increased risk of blockage and reduction in volume Stratification, thermal warming Enhanced treatment Fences Undesirable Desirable Undesirable - - Signs Negative effects of safety signs Essential Educational - - Slopes Gentle Gentle Maintenance/ Management Safety equipment As flat as possible at normal depth margin Steep and deep to minimise land use Shallow for improved treatment Regular Regular Minimal Regular Infrequent Negative impacts Essential SR R. 4.0

108 SR R. 4.0

109 9. Stormwater Ownership and Management This chapter provides an overview of drainage roles and responsibilities and the current situation for SUDS ownership and management and its implications for developers. This document is primarily a guide on how to use SUDS to ensure appropriate drainage is designed and built. However it is important to address the issue of ownership and management as the current division of responsibilities for drainage in the water industry make the issue of SUDS ownership a complex problem which is worth summarising. This chapter details the current situation, while acknowledging that parts of this section will rapidly become dated. 9.1 DRAINAGE RESPONSIBILITIES AND OWNERSHIP Drainage responsibilities involve a number of different stakeholders including: local authorities the environmental regulator sewerage undertakers highways authorities private land-owners Although good SUDS design can be achieved to meet the various needs of the community and environment, the ownership and maintenance of SUDS systems will influence the design. The following sections summarise the responsibilities of various stakeholder organisations, and this will inform the developer about the current SUDS ownership issues. It encourages an early consideration of ownership issues in the planning and design of the site. 9.2 LOCAL AUTHORITY DRAINAGE RESPONSIBILITIES Local authorities act as planning and building control authorities and also have responsibilities for local roads and their drainage, public landscaping, and land drainage. Planning authorities implement development policy and approve new development, including drainage. Their role is essential in ensuring that SUDS are incorporated into new developments. In Scotland, the local authority also has the primary flood management responsibility. Local authority-led Flood Appraisal Groups co-ordinate actions where catchments cover several separate local authority areas. Local authorities are responsible for administering the Building Regulations via their approved inspectors which includes drainage construction Ownership of SUDS Public open space is normally the responsibility of the local authority. Recently, management companies have been created to manage public open space. It is therefore possible that the SUDS components, which are located in these areas, might also become the responsibility of these organisations. However the responsibilities of these organisations are effectively underwritten by the local authority if the company becomes insolvent. SR R. 4.0

110 The mechanism for managing SUDS by the local authority is currently quite difficult due to fiscal rules within which authorities must operate. A commuted sum is the normal method of addressing the liability for maintaining the adopted SUDS component(s). Where land ownership, or maintenance responsibility is assumed by the local authority, it is taken under a Section 106 Town and Country Planning Act Agreement. Some SUDS, relating to Highways can be covered under the Highways Act ENVIRONMENTAL REGULATORS DRAINAGE RESPONSIBILITY In England and Wales, the Environment Agency is an executive agency of the Department of Environment, Food and Rural Affairs (Defra). It has wide-ranging responsibilities including the management of water resources, control of pollution in inland, estuarial and coastal waters, and flood defence. The principal duty of the Agency is to contribute towards the achievement of sustainable development. In carrying out all its functions, the Agency is subject to general duties to protect and enhance the environment and promote recreation. The Agency has general flood defence and land drainage powers, including a supervisory duty over all flooding (but primarily in respect of Main rivers). It is responsible for recommending planning conditions (discharge consents) relating to stormwater runoff. The Rivers Agency is the equivalent organisation responsible for watercourses in Northern Ireland. In Scotland, this is a local authority function. SEPA has equivalent powers for the control of pollution only. To meet statutory duties related to securing the proper use of water resources and as part of its role as statutory consultee to the land-use planning process, the Agency provides guidance and recommendations on the potential for pollution within a catchment and on good practice to prevent water pollution. The Agency is the competent authority for implementation of the Water Framework Directive, which requires that all discharges obtain consents or authorisation from the relevant body. Moreover the Agency has powers to serve notices requiring works to be carried out to protect watercourses and groundwater quality. The Scottish Environmental Protection Agency (SEPA) and the Northern Ireland Environment and Heritage Services (EHS) have similar powers. All these regulators have policies that promote SUDS as the preferred solution for drainage of surface water runoff, including roof water, for all proposed developments. In general, they will not serve prohibition notices (and the associated requirement to apply for consent) in relation to the quality of stormwater discharges. However, conditional prohibition notices may be served prior to construction to ensure that the drainage system is constructed to the agreed design Ownership of SUDS The Environment Agency will not take ownership of any drainage system, whether SUDS are involved or not. 9.4 SEWERAGE UNDERTAKERS There are nine water and sewerage companies across England, with individual companies serving Scotland (Scottish Water), Wales (Welsh Water) and Northern Ireland (Northern Ireland Water Service). They are responsible for both water supply SR R. 4.0

111 and sewerage. In addition, there are a number of water-only supply companies who do not have sewerage responsibilities. Figure 9.1 shows the locations of all the sewerage undertakers across the UK. The main duty of a sewerage undertaker under the Water Industry Act, 1991 is to provide a public sewer connection to be used for drainage of domestic premises in its area (if served notice by the owner or the occupier of the premise, and/or the local planning authority, and/or a development board or corporation). This process is known as requisitioning. The Act also allows a highway authority to enter into an agreement with the sewerage undertaker to use its sewers for conveying surface water from roads repairable by the authority. Similarly, the sewerage undertaker can enter into an agreement with the highway authority to use the highway drainage to carry surface water from premises or streets. The Act prohibits either party from refusing to enter into or consent to such an agreement on unreasonable grounds. In Scotland, the Water Environment and Water Services (Scotland) Act 2003 amended the Sewerage (Scotland) Act 1968 to include a definition of SUDS, thus giving public SUDS the same legal status as traditional sewers. The Act gives responsibility to Scottish Water for the adoption and maintenance of public SUDS in Scotland. The definition does not include private SUDS that are located entirely within the curtilage of a property, or SUDS that convey road drainage only. Sewerage undertakers issue technical approval of the parts of the proposed sewerage infrastructure that they will adopt / vest and assume maintenance responsibility from the developer. Water companies have a wide range of environmental policies through which they seek to minimise the environmental impact and maximise the sustainability of their core activities. They have a statutory duty to promote the efficient use of water, environmental good practice and the use of cost-effective solutions Ownership of SUDS Scottish Water will normally only vest Detention Ponds and Detention Basins. Other SUDS components are either deemed to be the domain of Highways (Filter trenches, Pervious pavements, Swales) as they are primarily associated with roads and parking. The revised version of Sewers for Scotland 2 nd Edition (WRc, expected in 2005) does make an emphasis on a holistic drainage design, with an expectation that SUDS will be provided for private properties and highways and vested with the appropriate owner. In addition there is an emphasis on maintenance and rehabilitation of SUDS systems with vesting being implemented up to 12 months after completion of construction. The situation in Northern Ireland is very similar to England and Wales in that there is no legal provision for Water Services to own such structures. However, an investigation is currently taking place to look into how SUDS can be adopted in Northern Ireland. It is being driven by the Northern Ireland Environment and Heritage Service and involves all relevant interested parties. Water Services is undergoing a major reorganisation with a change in its status. It can therefore be anticipated that, like Scotland, some progress can be expected to be made in SR R. 4.0

112 Figure 9.1 UK sewerage undertakers In England and Wales the Sewerage Undertakers have taken slightly varying positions on adopting SUDS, but in general the consensus is that the Water Resources Act 1991 defines a sewer as being a pipe and that there are too many issues that need addressing for SUDS to be considered for adoption under the current drainage legislative framework. For example the right to connect is seen as an obstacle to the widespread adoption of SUDS. In principle, however, there is general agreement amongst all stakeholders that SUDS are best practice drainage and this is reflected in the Interim Code of practice document produced by the National Suds Working Group in SR R. 4.0

113 Undertakers have defined their position on SUDS in this document. A key component relating to potential acceptance of a SUDS component is that it should have an outfall or overflow pipe. This is summarised in Table 9.1. UKWIR, their research organisation, is carrying out a number of research studies in anticipation of being required to adopt using SUDS systems. 9.5 HIGHWAY AUTHORITIES DRAINAGE RESPONSIBILITIES Highway authorities (both local highway authorities and the Highways Agency) have the power to construct, adopt and maintain highway drainage infrastructure. Each authority sets down standards which developers must follow throughout the construction process to ensure that adoptable roads are of satisfactory construction, safe for the public to use and able to be easily maintained. Effective road drainage is fundamentally important to road safety and to the integrity and structural stability of the road, including its footways, verges and margins. When considering construction consent applications, local authorities will want to be satisfied that SUDS employed in particular locations meet their road drainage requirements and will not require high levels of maintenance Ownership of SUDS The Highways Agency is quite proactive in research and the use of SUDS systems, though there is an emphasis on these components being referred to as vegetated drainage systems and not sustainable drainage. This is due to the belief that the sediment is likely to be sufficiently contaminated that it will need to be treated as being special waste. Local highways authorities do not seem to have taken a stand on using SUDS systems, but once the position of the Sewerage Undertakers is resolved, co-ordination with the local highways authority on drainage of developments will result in a more defined position having to be taken. 9.6 HOME OWNER / PRIVATE (IN-CURTILAGE) SUDS OWNERSHIP Homeowners are generally responsible for drainage within the curtilage of their properties. Ownership of this land may change through the development process, with a landowner selling the land to a developer and this then being sold to the house owner. Each party must therefore be made aware of any drainage constraints in terms of land use restrictions or maintenance requirements and relevant drainage details should be included within house sale documentation. Although there may be stipulations regarding the need to maintain aspects of the property to ensure the SUDS component(s) remain effective, in practice the risks related to change of use of some SUDS is high. Prevention and rectification of such changes may not be practical in that monitoring of all properties is virtually impossible. 9.7 CONSERVATION ORGANISATIONS DRAINAGE RESPONSIBILITIES English Nature is the government body in England whose purpose is to promote the conservation of wildlife and natural features. Powers have been given to them via several Acts of Parliament, and their duties now include the following: advising government, organisations and individuals on nature conservation; SR R. 4.0

114 designating the most important areas for wildlife and natural features, e.g. Sites of Special Scientific Interest (SSSI s), and securing management of these sites; implementing EC directives on nature conservation and biodiversity; supporting and undertaking research. The Countryside Council for Wales (CCW), Scottish Natural Heritage (SNH), and the Environment and Heritage Service of Northern Ireland are the relevant bodies for other parts of the UK. These organisations should be consulted during the planning process for any major development so that issues relating to wildlife and conservation can be addressed. This is likely to influence the design of SUDS systems. 9.8 SUMMARY OF SUDS ADOPTION BY DRAINAGE ORGANISATIONS Table 9.1 summarises the current position of SUDS adoption by the various organisations with drainage responsibilities. The table should be read taking into account the following explanations. Sewerage Undertakers: The adoption of SUDS is subject to resolution of what is seen as a number of legal constraints. In addition all the components are required to have outfalls to qualify as SUDS. Scottish Water: Although all SUDS except two are defined as not being adoptable in accordance with Sewers for Scotland 2 nd Edition (WRc, expected in 2005), negotiations with regard to sites which are deemed to have a high risk of pollution can result in other SUDS components being vested. Highways Agency: Vegetated open channels, ponds and wetlands are being used. Swales are designed rather differently to SUDS manuals with a focus on safety taking into account the risks relating to fast moving traffic. They are officially not recognised as SUDS components. Private owner: Theoretically private owners can be responsible for any form of SUDS component. However, in practice, there are a number of SUDS components that will not normally be used within the private curtilage due to the risk of change of use. SR R. 4.0

115 Table 9.1 SUDS ownership/maintenance by drainage organisation status as of 2004/2005 SUDS component Local authority Highways Authorities Water Butts, stormwater use systems Pervious pavement Filter drains Filter strips Swales Detention ponds Retention ponds Wetlands Detention basins Soakaways Infiltration trenches Infiltration basins Green roofs Bio-retention areas Underground storage Proprietary drainage units Pipes Can a component be adopted: yes no Scottish Water Sewerage U taker NI Water Service Private owner SR R. 4.0

116 SR R. 4.0

117 10. References British Council for Offices. (2003). Green roofs, research advice note, Corporation of London, London, UK CIRIA and HR Wallingford, (to be published). SUDS updated guidance on technical design and construction, Report RP697, CIRIA, London CIRIA, (2004). Interim Code of Practice for Sustainable Drainage Systems, CIRIA, London Countryside and Rights of Way Act, (2000). The Stationary Office, London Defra, (2000). Flood and Coastal Defence Project Appraisal Guidance 3 economic appraisal, Defra, London Defra, (2004). PPS23, Planning and Pollution Control. Defra, London. Department of the Environment and Department of Transport, (1992). Residential Roads and Footpaths, Design Bulletin 32, 2 nd Edition, HMSO, London Department of the Environment, Transport and the Regions, (2000). Housing, Planning Policy Guidance Note 3, Stationary Office, London Department of the Environment, Transport and the Regions, (2001). Development and flood risk, Planning Policy Guidance Note 25, Stationary Office, London DOE, (1981). The Wallingford Procedure, HR Wallingford Department of the Environment, (Currently under public consultation stage, comments by 29 April 2005), Planning Policy Statement 15: Planning and Flood Risk PPS15, Belfast, Northern Ireland DTLR, (2000). The Building Regulations, Part H: Drainage and Waste Disposal, The Stationary Office, 2002 Edition DTLR, (2002). Planning for Open space, Sport and Recreation, Planning Policy Guidance Notes 17, The Stationary Office, London Environment Agency, (2004). Preliminary rainfall runoff management for developments, R&D Technical Report W5-074/A, Environment Agency, UK Environment Agency, (2003). UK Climate Impacts Programme 2002 Climate Change Scenarios: Implementation for Floods and Coastal Defence: Guidance for Users, R&D Technical Report W5B-029/TR, Environment Agency, UK European Union, (2000). Directive 2000/60/EC of the European Parliament and the Council establishing a framework for the Community action in the field of water policy (Water Framework Directive) Hall, M.J., Hockin, D.L. and Ellis, J.B., (1993). Design of Flood Storage Reservoirs, CIRIA, UK SR R. 4.0

118 Hewlett, H.W.M., Boorman, L.A. and Bramley, M.E., (1987). Design of reinforced grass waterways, CIRIA Report 116, CIRIA, London Highways Act, (1980). HMSO, London Highways Agency, (1998). Manual of contract documents for highway works - Volume 1: Specification for highway works, HMSO, UK HR Wallingford, (2003). Maximising the Ecological Benefits of Sustainable Drainage Schemes, SR Report 625, HR Wallingford, Wallingford, UK Institute of Highway Incorporated Engineers, (2002). Home Zone - design guidelines, 1 st Edition, Institution of Highway Incorporated Engineers, UK Institute of Hydrology, (1975). Flood Studies Report, Institute of Hydrology, Wallingford, UK Institute of Hydrology, (1994). Flood Estimation for small catchments, Report 124, Institute of Hydrology, Wallingford, UK Institute of Hydrology, (1999). Flood Estimation Handbook (FEH), 5 Volumes, Institute of Hydrology, Wallingford, UK Kellagher, R.B.B., (2004). Drainage of Development Sites: A guide, CIRIA Report X108, CIRIA, London MAFF, (2000). Flood and Coastal Defence Project Appraisal Guidance 4 Approaches to Risk, MAFF, London MAFF, (1981). The design of field drainage pipe systems, Report 345, MAFF, London Martin, P, Turner, B, Dell, J, Payne, J, Elliot, C and Reed, B., (2001). Sustainable urban drainage systems best practice manual, Report C523, CIRIA, London Martin, P, Turner, B, Waddington, K, Dell, J, Pratt, C, Campbell, N, Payne, J and Reed, B (2000a). Sustainable urban drainage systems design manual for England and Wales, Report C522, CIRIA, London Martin, P, Turner, B, Waddington, K, Pratt, C, Campbell, N, Payne, J and Reed, B., (2000b). Sustainable urban drainage systems design manual for Scotland and Northern Ireland, Report C521, CIRIA, London Masters-Williams, H., Heap, A., Kitts, H., Greenshaw, L., Davies, S., Fisher, P., Hendrie, M., and Owens, D., (2001). Control of water pollution from construction sites, CIRIA Report C532, CIRIA, London National Assembly for Wales, (2004). Development and Flood Risk, Planning Guidance (Wales), Technical Advice Note (Wales) 15, Cardiff, TSO, 7pp National building specification landscape specification ( National Playing Fields Association, (2001). The Six Acre Standard: Minimum standards for outdoor playing space, National Playing Fields Association SR R. 4.0

119 National SUDS Working Group, (2004). Interim Code of Practice for Sustainable Drainage Systems, National SUDS Working Group ( National SUDS Working Group, (2003). Framework Document, National SUDS Working Group NERC, (1977). The estimation of low return period flows FSSR 2, Flood Studies Supplementary Report, Institute of Hydrology NERC, (1983). Review of regional growth curves FSSR 14, Flood Studies Supplementary Report, Institute of Hydrology NERC, (1985). The FSR rainfall-runoff model parameter estimation equations updated FSSR 16, Flood Studies Supplementary Report, Institute of Hydrology Occupiers Liability Act, (1957 and 1984). The Stationary Office, London ODPM, (unknown). Assessing needs and opportunities: Planning Policy Guidance 17 companion document, ODPM, hcsp Oxford University Press, (1989). Oxford English Dictionary, 2 nd University Press, Oxford, UK Edition, Oxford Pratt, C, Wilson, S, Cooper, P., (2002). Source control using constructed pervious surfaces Hydraulic, structural and water quality issues, C582, CIRIA, London Scottish Executive, (2004). Planning and Flooding, National Planning Policy Guidance 7 NPPG7, Edinburgh, Scottish Office Development Department, 31pp Scottish Executive, (2001). Planning and Sustainable Urban Drainage Systems, Planning Advice Note 61, Scottish Executive Development Department Scottish Executive, (2003). Planning and Open Space, Planning Advice Note 65, Scottish Executive Development Department Scottish Office, (1996). National Planning Policy Guidance 11: Sport, Physical Recreation and Open Space, Scottish Office Town and Country Planning Act, (1990). The Stationary Office, London UKWIR, (2004). Water for Scotland, WRc, UKWilson, S, Bray, R, Cooper, P., (2004). Sustainable drainage systems Hydraulic, structural and water quality advice, Report C609, CIRIA, London WRc, (2001). Sewers for adoption, 5 th Edition, WRc, UK WRc, (expected in 2005). Sewers for Scotland, 2nd Edition (Consultation Draft), WRc, UK SR R. 4.0

120 Additional references (not cited in text) Abbott, C.L, Comino, L., (2001). In situ performance monitoring of an infiltration drainage system and field testing of current design procedures, Journal of Water and Environmental Management, Vol. 15, July/August, p Bettess, R., (1996). Infiltration drainage manual of good practice, Report 156, CIRIA, London, pp107 BS EN :2000 Gravity drainage systems outside buildings Part 3, Roof drainage, layout and calculation, BSI BS EN 752-2:1997 Drain and sewer systems outside buildings part 2: Performance requirements, BSI BS EN 752-4:1998 Drain and sewer systems outside building - Part 4: Hydraulic design and environmental considerations, BSI Building Research Establishment, (1986). Soakaway design, Digest 365, Building Research Establishment, Garston, UK CIRIA, (1994). Control of pollution from highway drainage discharges, Report 142, reprint 1997, CIRIA, London, pp152 Comino, L., (2000). InfoWorks CS Modelling a permeable pavement as an infiltration system, Report IT 488, HR Wallingford, Wallingford, UK DETR, (undated). Better places to live by design: a companion guide to PPG3, pdf Dublin Drainage, (2004) Greater Dublin Strategic Drainage Study: Regional Policies Volume 2 New development, Dublin Drainage, Dublin, Ireland Environment Agency, (2003). Guidance manual for constructed wetlands, R&D Technical Report P2-159/TR2, Environment Agency, UK Hall, M. J, Hockin, D.L, Ellis, J.B., (1993). Design of flood storage reservoirs, Book 14, CIRIA, London Leggett, D, Brown, R, Brewer, D, Stanfield, G and Holliday, E., (2001). Rainwater and greywater use in buildings best practice guidance, Report C539, CIRIA, London Shaffer, P, Elliot, C, Reed, J, Holmes, J and Ward, M., (2004). Model agreements for sustainable drainage systems, Report C625, CIRIA, London UKWIR, (1998). Civil Engineering Specification for the Water Industry, 5 th Edition, WRc, UK WRc, (2001) Sewers for Scotland, 1 st Edition, WRc, UK SR R. 4.0

121 Appendices SR 666 R. 4.0

122 SR 666 R. 4.0

123 Appendix 1 Greenfield runoff estimation Appendix 1 provides information with regards to runoff estimation from greenfield areas. It also provides all the relevant values and the background to the use of the formulae. A1.1. Greenfield peak rate of runoff estimation The Institute of Hydrology Report No. 124 was published in 1994 and describes research on flood estimation for small catchments. The research was based on 71 small rural catchments (<25 km 2 ), some of which were additional to the original 1975 FSR catchment data set. A new regression equation was produced to calculate QBAR rural, the mean annual flood. QBAR rural is estimated from the equation: QBAR rural = AREA 0.89 SAAR 1.17 SPR 2.17 (A1.1) Where: QBAR rural = the mean annual flood flow from a rural catchment in m 3 /s AREA = the area of the catchment in km 2. SAAR = the standard average annual rainfall (for the period 1941 to 1970) in mm. This map is available with the Flood Studies Report or with the Wallingford Procedure. SPR = SPR value of the SOIL class, which is a composite index determined from soil survey maps that accompany the Flood Studies Report and the WRAP map of the Wallingford Procedure. QBAR rural can be factored by the regional growth curve to produce peak flood flows for other return periods. The mean annual flood approximates to a return period of around 2.3 years. The Flood Estimation Handbook (FEH), which was brought out in 1999, uses a revised soil index called HOST. These soil classification also have an SPR value. This means that the peak flow rate estimation formula can be applied using either HOST or SOIL classes. Purists would probably expressed concern as the correlation basis for the formula was carried out against the SOIL classes, and because of the HOST SPR values were not derived in the same way. The 5 SOIL classes and their respective SPR values are detailed in Table A1.1. Table A1.1 Soil classes SPR values SOIL class SPR value The soil map of the Wallingford Procedure was based on FSR and was produced as an A0 plan. It has been reduced to A4 for inclusion in this appendix. It should be noted that the Wallingford Procedure values for SPR were the original values from FSR which are slightly different to the values in Table A1.1 which were produced as a revision after the Wallingford Procedure was issued. The use of the SOIL class for any site must be treated with caution and only used for initial assessment of the catchment runoff characteristics. A soil survey is strongly recommended to confirm the mapping information. This is important because the soil class is the most significant parameter for this formula. Figure A1.1 is the SOIL (WRAP) map for UK from the Wallingford procedure. SR 666 A1.1 R. 4.0

124 Figure A1.1 SOIL classes map of UK SR 666 A1.2 R. 4.0